[llvm] [llvm][Docs] Clarify the process for requesting a merge on your behalf (PR #124154)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> I am not sure if it is possible to make a bot that suggests this when

I'm going to find out! - https://discourse.llvm.org/t/rfc-encouraging-reviewers-to-merge-on-behalf-of-pr-authors-without-commit-access/84180/14?u=davidspickett

https://github.com/llvm/llvm-project/pull/124154