[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:

Although I _could_ merge this, I want to wait until I get an approval from a 'docs owner'. Not sure who that'd be though.

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