[llvm][docs] Document how to ask for things other than commit access

[DavidSpickett]

This is the implicit process but useful to spell it out I think. To give folks more confidence in asking.

Inspired by https://discourse.llvm.org/t/request-github-issue-triage-permission-for-labeling-clang-issues/87126/1

[DavidSpickett]

Maybe we do document this but I looked in the triage guide and it wasn't there, I don't recall it anywhere else. Tell me if so, we can move/link one place to the other.