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

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

For these sort of PRs, where there's clear consensus on the approach in the corresponding Discourse thread, I'd add those involved in the discussion. I don't believe these sort of docs have a particular owner.

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