minor #9846 Added docs for Symfony Polyfill components (javiereguiluz)

This PR was squashed before being merged into the 2.8 branch (closes #9846).

Discussion
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Added docs for Symfony Polyfill components

This fixes #6052 but before completing it, I'd like to ask @wouterj @xabbuh and @nicolas-grekas if you like this proposal or if you expect something different for these components docs. Thanks!

Commits
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8c04898 Added docs for Symfony Polyfill components

components/polyfill_apcu.rst

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+.. index::
+    single: Polyfill
+    single: APC
+    single: Components; Polyfill
+
+The Symfony Polyfill / APCu Component
+=====================================
+
+    This component provides ``apcu_*`` functions and the ``APCUIterator`` class
+    to users of the legacy APC extension.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/polyfill-apcu
+
+Alternatively, you can clone the `<https://github.com/symfony/polyfill-apcu>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+Once this component is installed in your application, you can use the following
+classes and functions, no matter if the `PHP APCu extension`_ is installed or
+not in your server. The only requirement is to have installed at least the
+`legacy APC extension`_.
+
+Provided Classes
+~~~~~~~~~~~~~~~~
+
+* :phpclass:`APCUIterator`
+
+Provided Functions
+~~~~~~~~~~~~~~~~~~
+
+* :phpfunction:`apcu_add`
+* :phpfunction:`apcu_delete`
+* :phpfunction:`apcu_exists`
+* :phpfunction:`apcu_fetch`
+* :phpfunction:`apcu_store`
+* :phpfunction:`apcu_cache_info`
+* :phpfunction:`apcu_cas`
+* :phpfunction:`apcu_clear_cache`
+* :phpfunction:`apcu_dec`
+* :phpfunction:`apcu_inc`
+* :phpfunction:`apcu_sma_info`
+
+.. _`PHP APCu extension`: https://secure.php.net/manual/en/book.apcu.php
+.. _`legacy APC extension`: https://secure.php.net/manual/en/book.apc.php

components/polyfill_ctype.rst

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+.. index::
+    single: Polyfill
+    single: Ctype
+    single: Components; Polyfill
+
+The Symfony Polyfill / Ctype Component
+======================================
+
+    This component provides ``ctype_*`` functions to users who run PHP versions
+    without the ctype extension.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/polyfill-ctype
+
+Alternatively, you can clone the `<https://github.com/symfony/polyfill-ctype>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+Once this component is installed in your application, you can use the following
+functions, no matter if the `PHP Ctype extension`_ is installed or not in your
+server.
+
+Provided Functions
+~~~~~~~~~~~~~~~~~~
+
+* :phpfunction:`ctype_alnum`
+* :phpfunction:`ctype_alpha`
+* :phpfunction:`ctype_cntrl`
+* :phpfunction:`ctype_digit`
+* :phpfunction:`ctype_graph`
+* :phpfunction:`ctype_lower`
+* :phpfunction:`ctype_print`
+* :phpfunction:`ctype_punct`
+* :phpfunction:`ctype_space`
+* :phpfunction:`ctype_upper`
+* :phpfunction:`ctype_xdigit`
+
+.. _`PHP Ctype extension`: https://secure.php.net/manual/en/book.ctype.php

components/polyfill_iconv.rst

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+.. index::
+    single: Polyfill
+    single: Iconv
+    single: Components; Polyfill
+
+The Symfony Polyfill / Iconv Component
+======================================
+
+    This component provides a native PHP implementation of the ``iconv_*``
+    functions to users who run PHP versions without the ``iconv`` extension.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/polyfill-iconv
+
+Alternatively, you can clone the `<https://github.com/symfony/polyfill-iconv>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+Once this component is installed in your application, you can use the following
+constants and functions, no matter if the `PHP iconv extension`_ is installed or
+not in your server. The only function not implemented in this component is
+:phpfunction:`ob_iconv_handler`.
+
+Provided Constants
+~~~~~~~~~~~~~~~~~~
+
+* ``ICONV_IMPL`` (value = ``'Symfony'``)
+* ``ICONV_VERSION`` (value = ``'1.0'``)
+* ``ICONV_MIME_DECODE_STRICT`` (value = ``1``)
+* ``ICONV_MIME_DECODE_CONTINUE_ON_ERROR`` (value = ``2``)
+
+Provided Functions
+~~~~~~~~~~~~~~~~~~
+
+These functions are always available:
+
+* :phpfunction:`iconv`
+* :phpfunction:`iconv_get_encoding`
+* :phpfunction:`iconv_set_encoding`
+* :phpfunction:`iconv_mime_encode`
+* :phpfunction:`iconv_mime_decode_headers`
+
+These functions are only available when the ``mbstring`` or the ``xml``
+extension are installed:
+
+* :phpfunction:`iconv_strlen`
+* :phpfunction:`iconv_strpos`
+* :phpfunction:`iconv_strrpos`
+* :phpfunction:`iconv_substr`
+* :phpfunction:`iconv_mime_decode`
+
+.. _`PHP iconv extension`: https://secure.php.net/manual/en/book.iconv.php
+.. _`mbstring`: https://secure.php.net/manual/en/book.mbstring.php
+.. _`xml`: https://secure.php.net/manual/en/book.xml.php

components/polyfill_intl_grapheme.rst

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+.. index::
+    single: Polyfill
+    single: Intl
+    single: Components; Polyfill
+
+The Symfony Polyfill / Intl Grapheme Component
+==============================================
+
+    This component provides a partial, native PHP implementation of the
+    ``grapheme_*`` functions to users who run PHP versions without the ``intl``
+    extension.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/polyfill-intl-grapheme
+
+Alternatively, you can clone the `<https://github.com/symfony/polyfill-intl-grapheme>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+Once this component is installed in your application, you can use the following
+constants and functions, no matter if the `PHP intl extension`_ is installed or
+not in your server.
+
+Provided Constants
+~~~~~~~~~~~~~~~~~~
+
+* ``GRAPHEME_EXTR_COUNT`` (value = ``0``)
+* ``GRAPHEME_EXTR_MAXBYTES`` (value = ``1``)
+* ``GRAPHEME_EXTR_MAXCHARS`` (value = ``2``)
+
+Provided Functions
+~~~~~~~~~~~~~~~~~~
+
+* :phpfunction:`grapheme_extract`
+* :phpfunction:`grapheme_stripos`
+* :phpfunction:`grapheme_stristr`
+* :phpfunction:`grapheme_strlen`
+* :phpfunction:`grapheme_strpos`
+* :phpfunction:`grapheme_strripos`
+* :phpfunction:`grapheme_strrpos`
+* :phpfunction:`grapheme_strstr`
+* :phpfunction:`grapheme_substr`
+
+.. seealso::
+
+    The :doc:`polyfill-intl-icu </components/polyfill_intl_icu>` and
+    :doc:`polyfill-intl-normalizer </components/polyfill_intl_normalizer>`
+    components provide polyfills for other classes and functions related to the
+    Intl PHP extension.
+
+.. _`PHP intl extension`: https://secure.php.net/manual/en/book.intl.php

components/polyfill_intl_icu.rst

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+.. index::
+    single: Polyfill
+    single: ICU
+    single: Components; Polyfill
+
+The Symfony Polyfill / Intl ICU Component
+=========================================
+
+    This component provides a native PHP implementation of several Intl
+    functions and classes to users who run PHP versions without the ``intl``
+    extension.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/polyfill-intl-icu
+
+Alternatively, you can clone the `<https://github.com/symfony/polyfill-intl-icu>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+Once this component is installed in your application, you can use the following
+classes and functions, no matter if the `PHP intl extension`_ is installed or
+not in your server.
+
+Provided Classes
+~~~~~~~~~~~~~~~~
+
+* :phpclass:`Collator`
+* :phpclass:`IntlDateFormatter`
+* :phpclass:`Locale`
+* :phpclass:`NumberFormatter`
+
+Provided Functions
+~~~~~~~~~~~~~~~~~~
+
+* :phpfunction:`intl_error_name`
+* :phpfunction:`intl_get_error_code`
+* :phpfunction:`intl_get_error_message`
+* :phpfunction:`intl_is_failure`
+
+.. seealso::
+
+    The :doc:`polyfill-intl-grapheme </components/polyfill_intl_grapheme>` and
+    :doc:`polyfill-intl-normalizer </components/polyfill_intl_normalizer>`
+    components provide polyfills for other classes and functions related to the
+    Intl PHP extension.
+
+.. _`PHP intl extension`: https://secure.php.net/manual/en/book.intl.php

components/polyfill_intl_normalizer.rst

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+.. index::
+    single: Polyfill
+    single: Normalizer
+    single: Components; Polyfill
+
+The Symfony Polyfill / Intl Normalizer Component
+================================================
+
+    This component provides a fallback implementation for the ``Normalizer``
+    class to users who run PHP versions without the ``intl`` extension.
+
+Installation
+------------
+
+.. code-block:: terminal
+
+    $ composer require symfony/polyfill-intl-normalizer
+
+Alternatively, you can clone the `<https://github.com/symfony/polyfill-intl-normalizer>`_ repository.
+
+.. include:: /components/require_autoload.rst.inc
+
+Usage
+-----
+
+Once this component is installed in your application, you can use the following
+classes and functions, no matter if the `PHP intl extension`_ is installed or
+not in your server.
+
+Provided Classes
+~~~~~~~~~~~~~~~~
+
+* :phpclass:`Normalizer`
+
+Provided Functions
+~~~~~~~~~~~~~~~~~~
+
+* :phpfunction:`normalizer_is_normalized`
+* :phpfunction:`normalizer_normalize`
+
+.. seealso::
+
+    The :doc:`polyfill-intl-grapheme </components/polyfill_intl_grapheme>` and
+    :doc:`polyfill-intl-icu </components/polyfill_intl_icu>` components provide
+    polyfills for other classes and functions related to the Intl PHP extension.
+
+.. _`PHP intl extension`: https://secure.php.net/manual/en/book.intl.php