Reintroduce PytestReturnNotNoneWarning (#13495) (#13527)
Since this warning is meant to be permanent, added proper documentation to the `assert` section in the docs.
Fixes #13477
(cherry picked from commit 53f05c44d9530c4ac5ce5804ef75fe61713d46d8)
Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>
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