Added a note about number key keycodes always being SDLK_0...SDLK_9 even on AZERTY layouts

See bug 3188 for more discussion

include/SDL_keycode.h

@@ -38,6 +38,9 @@
  *  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.
  */
 typedef Sint32 SDL_Keycode;