Update header comment

Author: Free-Pascal-meets-SDL-Website

units/sdlkeycode.inc

@@ -7,6 +7,9 @@
    *  layout of the keyboard.  These values include Unicode values representing
    *  the unmodified character that would be generated by pressing the key, or
    *  an SDLK_* constant for those keys that do not generate characters.
+   *
+   *  A special exception is the number keys at the top of the keyboard which
+   *  always map to SDLK_0...SDLK_9, regardless of layout.
    *}
 type
   PSDL_KeyCode = ^TSDL_KeyCode;