[DeveloperPolicy] Update information on commit authorship (#191220)
The sections on commit authorship have not been updated since the
transition to Github for PRs. This patch attempts to update them.
This is aimed at addressing questions recently raised in
https://discourse.llvm.org/t/attributing-a-commit-to-a-different-author/90490.
I guess this is technically a policy change, but does not change
anything *de facto* as far as I can tell.
---------
Co-authored-by: Petr Hosek <phosek@google.com>