Make ErrInvalidNaNValue a const and fix docs

* make ErrInvalidNaNValue a const to prevent changes
* mention FromNaN32ps can return ErrInvalidNaNValue with sNaN (0x7c01)

README.md

@@ -86,7 +86,7 @@ Fromfloat32(f32 float32) Float16   // Float16 number converted from f32 using IE
 
 FromNaN32ps(nan float32) (Float16, error)   // Float16 NaN without modifying quiet bit.
                                             // The "ps" suffix means "preserve signaling".
-                                            // Returns ErrInvalidNaNValue if nan isn't a NaN.
+                                            // Returns sNaN and ErrInvalidNaNValue if nan isn't a NaN.
                                  
 Frombits(b16 uint16) Float16       // Float16 number corresponding to b16 (IEEE 754 binary16 rep.)
 NaN() Float16                      // Float16 of IEEE 754 binary16 not-a-number

float16.go

@@ -118,6 +118,7 @@ func (e float16Error) Error() string { return string(e) }
 // qNaN because it sets quiet bit = 1, this can return both sNaN and qNaN.
 // If the result is infinity (sNaN with empty payload), then the
 // lowest bit of payload is set to make the result a NaN.
+// Returns ErrInvalidNaNValue and 0x7c01 (sNaN) if nan isn't IEEE 754 NaN.
 // This function was kept simple to be able to inline.
 func FromNaN32ps(nan float32) (Float16, error) {
 	const SNAN = Float16(uint16(0x7c01)) // signalling NaN

float16_test.go

@@ -336,6 +336,9 @@ func TestFromNaN32ps(t *testing.T) {
 	if err != float16.ErrInvalidNaNValue {
 		t.Errorf("FromNaN32ps: in float32(math.Pi) wanted err float16.ErrInvalidNaNValue, got err = %q", err)
 	}
+	if err.Error() != "float16: invalid NaN value, expected IEEE 754 NaN" {
+		t.Errorf("unexpected string value returned by err.Error() for ErrInvalidNaNValue: %s", err.Error())
+	}
 	if uint16(nan) != 0x7c01 { // signalling NaN
 		t.Errorf("FromNaN32ps: in float32(math.Pi) wanted nan = 0x7c01, got nan = 0x%04x", uint16(nan))
 	}