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

Related to:
https://discourse.llvm.org/t/forbidding-username-in-commits/86997

**Context:**

When we merge a commit including tag to another username (e.g.
`@<someUser>`), that account will receive an email / notification every
time that PR is cherry-picked and pushed to a fork, generating _a lot_
of spam in the user's inbox. As of today, there's no way of disabling
this on GitHub settings.

**This PR** documents this error in our developer policy, and reminds
contributors to avoid it.

**Next steps:** I'm a big believer that any policy that is not enforced
via automation is not enforced at all, so I'd love to see some kind of
check to prevent this error. However, there does not seem to be an
agreement to do that _yet_. I'll see if I can gather support for doing
that.

---------

Co-authored-by: Jakub Kuderski <[email protected]>

Author: davidmrdavid

llvm/docs/DeveloperPolicy.rst

@@ -413,6 +413,10 @@ Below are some guidelines about the format of the message itself:
   message self-explanatory. Note that such non-public links should not be
   included in the submitted code.
 
+* Avoid 'tagging' someone's username in your commits and PR descriptions
+  (e.g., `@<someUser>`), doing so results in that account receiving a notification
+  every time the commit is cherry-picked and/or pushed to a fork.
+
 LLVM uses a squash workflow for pull requests, so as the pull request evolves
 during review, it's important to update the pull request description over the
 course of a review. GitHub uses the initial commit message to create the pull