[llvm] [Docs] [Developer Policy] Document best practice of not tagging a username in commit messages and PR descriptions (PR #164328)

[David Justo via llvm-commits]

davidmrdavid wrote:

Thanks all Unfortunately `buildkite` is being flakey, and I don't have the permissions to retry those checks (as far as I can tell), so I'll update my branch  (git pull), _push no other changes_, and merge once the `buildkite` check passes.

https://github.com/llvm/llvm-project/pull/164328