[doc] Add note about `torch.flip` returning new tensor and not view. (#50041)
Summary:
Reference: https://github.com/pytorch/pytorch/issues/38271
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50041
Reviewed By: izdeby
Differential Revision: D25883870
Pulled By: mruberry
fbshipit-source-id: 33cc28a2176e98f2f29077958782291609c7999b
