Improved documentation

src/arm/Mul_test.go

@@ -34,6 +34,7 @@ func TestMultiplySubtract(t *testing.T) {
 		Code        uint32
 	}{
 		{arm.X0, arm.X1, arm.X2, arm.X3, 0x9B028C20},
+		{arm.X3, arm.X0, arm.X2, arm.X1, 0x9B028403},
 	}
 
 	for _, pattern := range usagePatterns {