Document main_menu state machine, name window constants

The new names for the constants (and newly named functions) give a much
better understanding of what the window system is trying to do.

include/window.h

@@ -1,6 +1,13 @@
 #ifndef GUARD_WINDOW_H
 #define GUARD_WINDOW_H
 
+// The tile ID for a transparent tile in the window tilesheet
+#define TRANSPARENT_TILE_NUMBER 0
+
+#define PIXEL_BUFFER_TRANSPARENT 0x00
+#define PIXEL_BUFFER_WHITE 0x11
+#define PIXEL_BUFFER_UNKNOWN 0xFF
+
 enum
 {
     WINDOW_BG,
@@ -73,6 +80,7 @@ extern u32 filler_03002F58;
 extern u32 filler_03002F5C;
 extern u32 filler_03002F64;
 
-extern u8 gUnknown_03002F60;
+// This global is set to TRANSPARENT_TILE_NUMBER and never changed.
+extern u8 gTransparentTileNumber;
 
 #endif // GUARD_WINDOW_H