[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.

Author: efriedma-quic

llvm/docs/LangRef.rst

@@ -2447,12 +2447,14 @@ example:
     memory access, I/O, or other synchronization.  The ``mustprogress``
     attribute is intended to model the requirements of the first section of
     [intro.progress] of the C++ Standard. As a consequence, a loop in a
-    function with the `mustprogress` attribute can be assumed to terminate if
+    function with the ``mustprogress`` attribute can be assumed to terminate if
     it does not interact with the environment in an observable way, and
-    terminating loops without side-effects can be removed. If a `mustprogress`
-    function does not satisfy this contract, the behavior is undefined.  This
-    attribute does not apply transitively to callees, but does apply to call
-    sites within the function. Note that `willreturn` implies `mustprogress`.
+    terminating loops without side-effects can be removed. If a ``mustprogress``
+    function does not satisfy this contract, the behavior is undefined. If a
+    ``mustprogress`` function calls a function not marked ``mustprogress``,
+    and that function never returns, the program is well-defined even if there
+    isn't any other observable progress.  Note that ``willreturn`` implies
+    ``mustprogress``.
 ``"warn-stack-size"="<threshold>"``
     This attribute sets a threshold to emit diagnostics once the frame size is
     known should the frame size exceed the specified value.  It takes one