docs: Add two footnotes about PEP 660 support (#3407)

docs/userguide/pyproject_config.rst

@@ -7,8 +7,8 @@ Configuring setuptools using ``pyproject.toml`` files
 .. note:: New in 61.0.0
 
 .. important::
-   For the time being, ``pip`` still might require a ``setup.py`` file
-   to support :doc:`editable installs <pip:cli/pip_install>`.
+   For the time being [#pep660-status]_, ``pip`` still might require a ``setup.py`` file
+   to support :doc:`editable installs <pip:cli/pip_install>` [#setupcfg-caveats]_.
 
    A simple script will suffice, for example:
 
@@ -214,6 +214,13 @@ however please keep in mind that all non-comment lines must conform with :pep:`5
 
 .. rubric:: Notes
 
+.. [#pep660-status] Editable install without ``setup.py`` will be supported in
+   future versions of ``setuptools``. Check https://github.com/pypa/setuptools/issues/2816 for detail.
+
+.. [#setupcfg-caveats] ``pip`` may allow editable install only with ``pyproject.toml``
+   and ``setup.cfg``. However, this behavior may not be consistent over various build
+   tools. Having a ``setup.py`` is still recommended if you rely on one of these tools.
+
 .. [#entry-points] Dynamic ``scripts`` and ``gui-scripts`` are a special case.
    When resolving these metadata keys, ``setuptools`` will look for
    ``tool.setuptool.dynamic.entry-points``, and use the values of the