[Easy][FSDP] Update full osd warning (#75109)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/75109
Clarify that only rank 0 receives a populated optimizer state dict to avoid confusion.
Test Plan: Imported from OSS
Reviewed By: mruberry
Differential Revision: D35319158
Pulled By: awgu
fbshipit-source-id: f8d5d4b5d94a6ffd89a42c0e63f40e920d3c07ad
(cherry picked from commit 7c1d719e3b3a74c560b12da49daecd5f5db19c12)
