[torch/elastic] Revise the note section of RendezvousHandler doc (#57723)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57723
Updated the note section of `RendezvousHandler`:
- Removed the experimental API warning.
- Recommended using the C10d Store instead of etcd for most users.
Test Plan: N/A
Reviewed By: kiukchung
Differential Revision: D28253828
fbshipit-source-id: c4f34dffd1a3cc132977029fe449b6d63ddc877b
