Update full backward hook doc with not-same-object note (#63245)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/61446
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63245
Reviewed By: ejguan
Differential Revision: D30352656
Pulled By: albanD
fbshipit-source-id: 7000ecb54a80f2da968ec7600b98574b608578ae
