Add a comment about use cases of util_*_infinity()

src/util/fpclassify.h

@@ -25,5 +25,10 @@ int util_isnan(double x);
 int util_isinf(float x);
 int util_isinf(double x);
 
+// The following functions are only used in testing code.
+// Don't use them in other -ffast-math code to avoid undefined behavior in
+// floating-point arithmetic where the compiler assumes that arguments and
+// results are not NaNs or +-Infs. For checking external librarie's return
+// values use the appropiated function above.
 float util_float_infinity();
 double util_double_infinity();