[LangRef] Document the de facto meaning of convergent

[foad]

Summary

The "convergent" attribute was originally documented as "cannot be made
control-dependent on any *additional* values", but in practice it now
prevents any change in control dependence.

[foad]

llvm/docs/LangRef.rst

1749

I found it hard to word this. Suggestions welcome!

[nhaehnle]

I think this is a reasonable compromise until we can land D147116, but maybe give a few more days for others to chime in.

cc @jlebar

[jlebar]

llvm/docs/LangRef.rst

1749

(GPT-4 sometimes gives reasonable rewording suggestions.)

1750

Do they depend on the *values*, or do they depend on the branch [taken/not-taken]ness?

1758

presumably you want to change this part ("on additional")

llvm/docs/LangRef.rst

``builtin``
     direct calls to functions that are declared with the ``nobuiltin``
     attribute.
 ``cold``
     This attribute indicates that this function is rarely called. When
     computing edge weights, basic blocks post-dominated by a cold
     function call are also considered to be cold; and, thus, given low
     weight.
 ``convergent``
-    In some parallel execution models, there exist operations that cannot be
-    made control-dependent on any additional values.  We call such operations
-    ``convergent``, and mark them with this attribute.
+    In some parallel execution models, there exist operations that implicitly
+    depend on the set of values upon which they are control-dependent.  We call
+    such operations ``convergent``, and mark them with this attribute.
 
     The ``convergent`` attribute may appear on functions or call/invoke
     instructions.  When it appears on a function, it indicates that calls to
-    this function should not be made control-dependent on additional values.
-    For example, the intrinsic ``llvm.nvvm.barrier0`` is ``convergent``, so
-    calls to this intrinsic cannot be made control-dependent on additional
-    values.
+    this function should not be made control-dependent on a different set of
+    values. For example, the intrinsic ``llvm.nvvm.barrier0`` is
+    ``convergent``, so calls to this intrinsic cannot be made control-dependent
+    on additional values.
 
     When it appears on a call/invoke, the ``convergent`` attribute indicates
     that we should treat the call as though we're calling a convergent
     function.  This is particularly useful on indirect calls; without this we
     may treat such calls as though the target is non-convergent.
 
     The optimizer may remove the ``convergent`` attribute on functions when it
     can prove that the function does not execute any convergent operations.