[spec] Clarifying note about canonical NaNs (WebAssembly#1614)

Author: rossberg

document/core/syntax/values.rst

@@ -140,6 +140,8 @@ An *arithmetic NaN*  is a floating-point value :math:`\pm\NAN(n)` with :math:`n
 .. note::
    In the abstract syntax, subnormals are distinguished by the leading 0 of the significand. The exponent of subnormals has the same value as the smallest possible exponent of a normal number. Only in the :ref:`binary representation <binary-float>` the exponent of a subnormal is encoded differently than the exponent of any normal number.
 
+   The notion of canonical NaN defined here is unrelated to the notion of canonical NaN that the |IEEE754|_ standard (Section 3.5.2) defines for decimal interchange formats.
+
 Conventions
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