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

[Chuanqi Xu via llvm-commits]

https://github.com/ChuanqiXu9 approved this pull request.

BTW, I am not sure if it is possible to make a bot that suggests this when:
- The pr is approved. And,
- The author didn't have commit access

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