[PyTorch Distributed] Update documentation about NCCL environment variables (#74006)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74006
updated recommendations about environment variables to use during debug
and performance tuning
Test Plan: `make html`
Reviewed By: rohan-varma
Differential Revision: D34767454
fbshipit-source-id: 08cd58469bf72b58702e50e82020fa19b43b5911
(cherry picked from commit ac7e6630f8043f85d3d16be17c6a8ad1ebb2990c)
