Add more details to explain rpc_backend_options arg in init_rpc
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/30855
Test Plan: Imported from OSS
Differential Revision: D18847529
Pulled By: mrshenli
fbshipit-source-id: b4f0d5797f3b41cce155b7821d6bd34b268bd24e
