Update sgn_FAC1

src/basic/math/bc39.sgn_FAC1.s

@@ -3,12 +3,16 @@
 ;; #LAYOUT# *   BASIC_0 #TAKE
 ;; #LAYOUT# *   *       #IGNORE
 
-;
+; This file is under the MIT license, it contains code released by Microsoft Corporation.
+; See LICENSE for more information.
+
 ; Math package - evaluate FAC1 sign, to FAC1
 ;
 ; Output:
 ; - FAC1 - 0 for FAC1 equal 0, 1 for FAC1 > 0, -1 for FAC1 < 0
 ;
+; This is verified to be identical to the original Microsoft implementation where it was named SGN.
+;
 ; See also:
 ; - [CM64] Computes Mapping the Commodore 64 - page 115
 ; - https://www.c64-wiki.com/wiki/Floating_point_arithmetic
@@ -17,4 +21,5 @@
 
 sgn_FAC1:
 	jsr sgn_FAC1_A
+
 	; Fallthrough to convert_A_to_FAC1