[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]

================
@@ -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 an email
+  every time the commit is cherry-picked and to a fork.
----------------
jh7370 wrote:

The "and" here seems misplaced?

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