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

[Tom Stellard via llvm-commits]

tstellar 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.

To do that you would need to add a step [here](https://github.com/llvm/llvm-project/blob/main/.github/workflows/new-prs.yml#L69) to check for that label and then skip the labler step if it exists.

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