Update doc about subsequent builds: options can be changed in build/CMakeCache.txt
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/23331
Test Plan: Imported from OSS
Differential Revision: D16517622
Pulled By: ezyang
fbshipit-source-id: d2d15b8bb2599b3b9abb7a1aa6bc91bfc0d8e5d0
