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

[Robert C. Seacord via llvm-commits]

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@@ -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
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rcseacord wrote:

I checked with the chair of the WG14 floating-point study group and received this response:

> Clause 7 is for all implementations, not just Annex F ones, so cannot require floating-point exceptions. See F.10.6.4. It’s implementation-defined behavior, but Annex F defines the behavior for implementations that support it.

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