[llvm] [LangRef] Try to clarify mustprogress wording. (PR #90510)

[Nikita Popov via llvm-commits]

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

LGTM

Agree that this is what we want -- and I've definitely seen confusion over the current wording.

https://github.com/llvm/llvm-project/pull/90510