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

[David Blaikie via llvm-commits]

dwblaikie wrote:

> Is the goal to not send any notifications if this label is applied to a PR?

Yes - that would be my preference/goal. If I'm not using the PR for review, as much as possible, it should be as if it never had a PR and was just committed - as far as the rest of the community is concerned.

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