DOC Update register_buffer/parameter docstring explaining None (#59015)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/40977
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59015
Reviewed By: ngimel
Differential Revision: D28797948
Pulled By: jbschlosser
fbshipit-source-id: 3bf60af5c1cfc5f1786b4975b48f093391374503
