DOC Improves shape documentation for *Flatten (#60980)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/60841
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60980
Reviewed By: VitalyFedyunin
Differential Revision: D29526650
Pulled By: jbschlosser
fbshipit-source-id: 2b4b0b84e0652c4cf3e9a48debb3b1bfe4e04b05
