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

[ofri frishman via llvm-commits]

ofri-frishman 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!

Yeah, looks good to me. I should probably go over these documents some more to better familiarize myself with all the aspects of contributing. I added a small comment regarding what I encountered.

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