[llvm] Document use of `skip-precommit-approval` label for non-review pull requests (PR #81053)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

As far as I'm aware, there hasn't been any work done on implementing this on the  subscription side? We should probably wire the label into the subscription job so that we don't end up sending extra notifications (which was part of the original RFC from what I understand), unless I'm missing something and that has already been done.

https://github.com/llvm/llvm-project/pull/81053