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

[Andy Kaylor 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
+is assumed to be set to "nearest", so halfway cases are rounded to the
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andykaylor wrote:

It's probably worth mentioning that this assumption is because of the default environment (with a link) and mentioning that constrained intrinsics can be used to avoid the assumption.

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