doc/Makefile: Update comment about sphinx entrypoint
https://github.com/sphinx-doc/sphinx/issues/4375 has been closed as
wontfix
. Update the comment in doc/Makefile that mentions it, based on
the discussion on the issue.
https://github.com/sphinx-doc/sphinx/issues/4375 has been closed as
wontfix
. Update the comment in doc/Makefile that mentions it, based on
the discussion on the issue.