minor #4255 Updated autoload standard to PSR-4. (phansys)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4255).

Discussion
----------

Updated autoload standard to PSR-4.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New feature?  | no
| Applies to    | 2.0+
| Tests pass?   | yes
| Fixed tickets | #3616

Commits
-------

d9a2f61 Updated autoload standard to PSR-4.

contributing/code/standards.rst

@@ -9,7 +9,7 @@ follow the same guidelines, and you should too.
 Remember that the main advantage of standards is that every piece of code
 looks and feels familiar, it's not about this or that being more readable.
 
-Symfony follows the standards defined in the `PSR-0`_, `PSR-1`_ and `PSR-2`_
+Symfony follows the standards defined in the `PSR-0`_, `PSR-1`_, `PSR-2`_ and `PSR-4`_
 documents.
 
 Since a picture - or some code - is worth a thousand words, here's a short
@@ -111,7 +111,7 @@ Structure
 
 * Define one class per file - this does not apply to private helper classes
   that are not intended to be instantiated from the outside and thus are not
-  concerned by the `PSR-0`_ standard;
+  concerned by the `PSR-0`_ and `PSR-4`_ autoload standards;
 
 * Declare class properties before methods;
 
@@ -185,3 +185,4 @@ License
 .. _`PSR-0`: http://www.php-fig.org/psr/psr-0/
 .. _`PSR-1`: http://www.php-fig.org/psr/psr-1/
 .. _`PSR-2`: http://www.php-fig.org/psr/psr-2/
+.. _`PSR-4`: http://www.php-fig.org/psr/psr-4/

cookbook/bundles/best_practices.rst

@@ -342,4 +342,4 @@ Learn more from the Cookbook
 
 * :doc:`/cookbook/bundles/extension`
 
-.. _standards: http://www.php-fig.org/psr/psr-0/
+.. _standards: http://www.php-fig.org/psr/psr-4/