[docs] Document our norms around reverts

This has come up a few times recently, and I was surprised to notice that we don't have anything in the docs.

This patch deliberately sticks to stuff that is uncontroversial in the community. Everything herein is thought to be widely agreed to by a large majority of the community.  A few things were noted and removed in review which failed this standard, if you spot anything else, please point it out.

Differential Revision: https://reviews.llvm.org/D99305