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.

src/money.c

@@ -169,7 +169,7 @@ void DrawMoneyBox(int amount, u8 x, u8 y)
 
     SetWindowTemplateFields(&template, 0, x + 1, y + 1, 10, 2, 15, 8);
     sMoneyBoxWindowId = AddWindow(&template);
-    FillWindowPixelBuffer(sMoneyBoxWindowId, 0);
+    FillWindowPixelBuffer(sMoneyBoxWindowId, PIXEL_BUFFER_TRANSPARENT);
     PutWindowTilemap(sMoneyBoxWindowId);
     CopyWindowToVram(sMoneyBoxWindowId, 1);
     PrintMoneyAmountInMoneyBoxWithBorder(sMoneyBoxWindowId, 0x214, 14, amount);