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

LLVM assumes round-to-nearest mode and sometimes performs constant-folding based on that assumption. This updates the language ref documentation for the rint and nearbyint intrinsics to mention that fact.

Author: RalfJung

llvm/docs/LangRef.rst

@@ -15754,7 +15754,11 @@ 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. Since LLVM assumes the
+:ref:`default floating-point environment <floatenv>`, the rounding mode is
+assumed to be set to "nearest", so halfway cases are rounded to the even
+integer. Use :ref:`Constrained Floating-Point Intrinsics <constrainedfp>`
+to avoid that assumption.
 
 .. _int_nearbyint:
 
@@ -15792,7 +15796,11 @@ Semantics:
 """"""""""
 
 This function returns the same values as the libm ``nearbyint``
-functions would, and handles error conditions in the same way.
+functions would, and handles error conditions in the same way. Since LLVM
+assumes the :ref:`default floating-point environment <floatenv>`, the rounding
+mode is assumed to be set to "nearest", so halfway cases are rounded to the even
+integer. Use :ref:`Constrained Floating-Point Intrinsics <constrainedfp>` to
+avoid that assumption.
 
 .. _int_round: