Add comments about adding new build options. (#22641)
Summary:
Also revert the change of cmake.py in
c97829d7011bd59d662f6af9c3a0ec302e7e75fc . The comments are added to
prevent future similar incidents in the future (which has occurred a couple of times in the past).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22641
Differential Revision: D16171763
Pulled By: ezyang
fbshipit-source-id: 5a65f9fbb3c1c798ebd25521932bfde0ad3d16fc
