[llvm] LangRef: rint, nearbyint: mention that default rounding mode is assumed (PR #77191)

[Ralf Jung via llvm-commits]

================
@@ -15751,7 +15751,9 @@ Semantics:
 """"""""""
 
 This function returns the same values as the libm ``rint`` functions
-would, and handles error conditions in the same way.
+would, and handles error conditions in the same way. The rounding mode
----------------
RalfJung wrote:

> In any case, I think the exception behavior doesn't need to be addressed by this patch.

:)

I don't have permissions in this repo, @andykaylor since you approved -- can you also do the merge or how does that work?

https://github.com/llvm/llvm-project/pull/77191