LangRef: Fix minimumnum/maximumnum nan handling phrasing (#139228)

Make this consistent with other operations with respect to
signaling nan quieting. This was specifying that quieting is
required, which is true for IEEE. Make this consistent with other
IR operations, and signaling nan quieting is possible but optional
in the case where there are two nan inputs.

This permits directly selecting the intrinsic to the hardware
instruction in the default floating-point environment for shaders.

Author: arsenm

llvm/docs/LangRef.rst

@@ -17200,12 +17200,14 @@ type.
 
 Semantics:
 """"""""""
-If both operands are NaNs (including sNaN), returns qNaN. If one operand
-is NaN (including sNaN) and another operand is a number, return the number.
-Otherwise returns the lesser of the two arguments. -0.0 is considered to
-be less than +0.0 for this intrinsic.
 
-Note that these are the semantics of minimumNumber specified in IEEE 754-2019.
+If both operands are NaNs (including sNaN), returns a :ref:`NaN <floatnan>`. If
+one operand is NaN (including sNaN) and another operand is a number,
+return the number.  Otherwise returns the lesser of the two
+arguments. -0.0 is considered to be less than +0.0 for this intrinsic.
+
+Note that these are the semantics of minimumNumber specified in
+IEEE-754-2019 with the usual :ref:`signaling NaN <floatnan>` exception.
 
 It has some differences with '``llvm.minnum.*``':
 1)'``llvm.minnum.*``' will return qNaN if either operand is sNaN.
@@ -17246,12 +17248,15 @@ type.
 
 Semantics:
 """"""""""
-If both operands are NaNs (including sNaN), returns qNaN. If one operand
-is NaN (including sNaN) and another operand is a number, return the number.
-Otherwise returns the greater of the two arguments. -0.0 is considered to
-be less than +0.0 for this intrinsic.
 
-Note that these are the semantics of maximumNumber specified in IEEE 754-2019.
+If both operands are NaNs (including sNaN), returns a
+:ref:`NaN <floatnan>`. If one operand is NaN (including sNaN) and
+another operand is a number, return the number.  Otherwise returns the
+greater of the two arguments. -0.0 is considered to be less than +0.0
+for this intrinsic.
+
+Note that these are the semantics of maximumNumber specified in
+IEEE-754-2019  with the usual :ref:`signaling NaN <floatnan>` exception.
 
 It has some differences with '``llvm.maxnum.*``':
 1)'``llvm.maxnum.*``' will return qNaN if either operand is sNaN.