[llvm] [llvm][docs] Extend docs on GitHub's "squash and merge" (PR #129497)

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

I also added a few more reviewers, mostly folks who have recently touched the update files. Please let me know if there's anyone else that we should include here. Thank you :)

https://github.com/llvm/llvm-project/pull/129497