docs: Note that :c:func: should no longer be used

Now that we can mark up function() automatically, there is no reason to use
:c:func: and every reason to avoid it. Adjust the documentation to reflect
that fact.

Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/doc-guide/sphinx.rst

···
 
 The func-name (e.g. ioctl) remains in the output but the ref-name changed from
 ``ioctl`` to ``VIDIOC_LOG_STATUS``. The index entry for this function is also
-changed to ``VIDIOC_LOG_STATUS`` and the function can now referenced by:
+changed to ``VIDIOC_LOG_STATUS``.
 
-.. code-block:: rst
-
-     :c:func:`VIDIOC_LOG_STATUS`
+Please note that there is no need to use ``c:func:`` to generate cross
+references to function documentation.  Due to some Sphinx extension magic,
+the documentation build system will automatically turn a reference to
+``function()`` into a cross reference if an index entry for the given
+function name exists.  If you see ``c:func:`` use in a kernel document,
+please feel free to remove it.
 
 
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