[BE][Docs] Improve dist.new_group doc (#55660)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55660
Noticed this doc was missing clarification on nccl env vars that
init_process_group docs have. Also, specify default behavior when backend=None
is passed in.
ghstack-source-id: 126251116
Test Plan: Ci
Reviewed By: SciPioneer
Differential Revision: D27672208
fbshipit-source-id: 2e79d297174e135173bceb059450ea267367bde4
