[llvm] [llvm][docs][nfc] Add a note on GitHub's "squash and merge" (PR #129497)

[David Spickett via llvm-commits]

DavidSpickett wrote:

This is good, but maybe the PR description needs to be updated since this is now more like moving the existing note into a place where people will actually see it.

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