[FSDP] s/local_state_dict/_local_state_dict (#73532)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73532
Also a few other small changes to documentation.
ghstack-source-id: 150334840
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D34524478
fbshipit-source-id: b80362e0b72422d2f7f8dcdf29e50841a2e9b253
(cherry picked from commit d5cc84ccd5abba02161439a2758b82043172bd72)
