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/battle_dome.c

@@ -3794,7 +3794,7 @@ static void sub_8190CD4(u8 taskId)
             for (i = windowId; i < windowId + 9; i++)
             {
                 CopyWindowToVram(i, 2);
-                FillWindowPixelBuffer(i, 0);
+                FillWindowPixelBuffer(i, PIXEL_BUFFER_TRANSPARENT);
             }
             gTasks[taskId].data[0] = 3;
             break;