Update docs for RPC async_execution (#45458)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/45458
Test Plan: Imported from OSS
Reviewed By: pritamdamania87
Differential Revision: D23973366
Pulled By: mrshenli
fbshipit-source-id: 3697f07fa972db21746aa25eaf461c1b93293f58
