Add documentation for c10d log levels (#73361)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73361
This PR adds the documentation for the newly introduced `TORCH_CPP_LOG_LEVEL` and how it can be used along with `TORCH_DISTRIBUTED_DEBUG` to adjust the log level of c10d.
ghstack-source-id: 149874995
Test Plan: Locally rendered and checked the documentation.
Reviewed By: rohan-varma
Differential Revision: D34452352
fbshipit-source-id: ecb54590f3030ddef9921a7152ca9f7fc9438345
(cherry picked from commit f4c7c6f3b27dbd3006686cf26a6e9e53cd2c8f09)
