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

[Mehdi Amini via llvm-commits]

================
@@ -103,6 +103,12 @@ ready to be committed. Specifically, once a patch is sent out for review, it
 needs an explicit approval before it is committed. Do not assume silent
 approval, or solicit objections to a patch with a deadline.
 
+.. note::
+   If you are using a Pull Request to get precommit CI results, but not
+   because it needs a precommit review, apply the `skip-precommit-approval
+   <https://github.com/llvm/llvm-project/labels?q=skip-precommit-approval>`
+   label to the PR.
----------------
joker-eph wrote:

```suggestion
   If you are using a Pull Request to get precommit CI results or for a revert, but
   not because it needs a precommit review, apply the `skip-precommit-approval
   <https://github.com/llvm/llvm-project/labels?q=skip-precommit-approval>`
   label to the PR.
```

(adding revert here)

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