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

Ensure it's clear that:

- Infinite loops in non-mustprogress functions are well-defined, even if
they're called by mustprogress functions.
- Infinite recursion in mustprogress functions is not well-defined.

Looking at D86233, it's clear this was the intent, but the "transitive"
wording is ambiguous. Instead, just explicitly state that infinite loops
written in non-mustprogress functions count as progress.