docs: Add a note about historical wheel use in requires (#3859)

Author: abravalheri

changelog.d/3859.doc.rst

@@ -0,0 +1,2 @@
+Added a note about historical presence of ``wheel``
+in ``build-system.requires``, in ``pyproject.toml``.

docs/userguide/quickstart.rst

@@ -56,6 +56,17 @@ containing a ``build-system`` section similar to the example below:
 This section declares what are your build system dependencies, and which
 library will be used to actually do the packaging.
 
+.. note::
+
+   Historically this documentation has unnecessarily listed ``wheel``
+   in the ``requires`` list, and many projects still do that. This is
+   not recommended. The backend automatically adds ``wheel`` dependency
+   when it is required, and listing it explicitly causes it to be
+   unnecessarily required for source distribution builds.
+   You should only include ``wheel`` in ``requires`` if you need to explicitly
+   access it during build time (e.g. if your project needs a ``setup.py``
+   script that imports ``wheel``).
+
 In addition to specifying a build system, you also will need to add
 some package information such as metadata, contents, dependencies, etc.
 This can be done in the same ``pyproject.toml`` [#beta]_ file,