Fix doc-gen warnings in RPC (#37666)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37666
Add `:orphan:` to avoid "WARNING: document isn't included in any toctree".
Test Plan: Imported from OSS
Differential Revision: D21351053
Pulled By: mrshenli
fbshipit-source-id: 6ff67c418fc1de410c7dc39ad9a0be5c30d07122
