[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:

> I'd add those involved in the discussion

I will do so next time, thank you for the guidance.

@nikic - can I'd appreciate a final pass now that the feedback is incorporated, so I can merge. Thank you!

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