[TensorPipe] Update documentation (#40222)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40222
Mention the TensorPipe agent in the RPC docs and give users the information they need to choose which agent to use.
ghstack-source-id: 106225711
Test Plan: Export to GitHub, build locally and try out the docs.
Differential Revision: D22116494
fbshipit-source-id: 30703ba8410c40f64e785f60d71dfd9faa8de4a1
