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

[Andrzej Warzyński via llvm-commits]

banach-space wrote:

@ofri-frishman , your recent PR prompted me to write this update. Could you take a look and tell us whether it makes things clearer? Thanks!

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