[DOC] improve docstring for Optimizer.state_dict (#63153)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63153
Fixes: https://github.com/pytorch/pytorch/issues/60121
Test Plan: Imported from OSS
Reviewed By: pbelevich
Differential Revision: D30629462
Pulled By: tugsbayasgalan
fbshipit-source-id: a9160e02ac53bb1a6219879747d73aae9ebe4d2f
