[SR] Update memory planner docs (#69559)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69559
We have a lot of special cases. Document them so they're easy to learn about.
ghstack-source-id: 145226542
Test Plan: Spell check? :)
Reviewed By: d1jang
Differential Revision: D32929416
fbshipit-source-id: 2362410f25a27cdb74a4939903446192cef61978
