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/clear_save_data_screen.c

@@ -90,7 +90,7 @@ static void Task_ClearSaveDataScreenYesNoChoice(u8 taskId)
     switch (Menu_ProcessInputNoWrapClearOnChoose())
     {
     case 0:
-        FillWindowPixelBuffer(0, 17);
+        FillWindowPixelBuffer(0, PIXEL_BUFFER_WHITE);
         AddTextPrinterParameterized(0, 1, gText_ClearingData, 0, 1, 0, 0);
         gTasks[taskId].func = Task_ClearSaveData;
         break;
@@ -203,7 +203,7 @@ static void InitClearSaveDataScreenWindows(void)
 {
     InitWindows(sClearSaveTextWindow);
     DeactivateAllTextPrinters();
-    FillWindowPixelBuffer(0, 0);
+    FillWindowPixelBuffer(0, PIXEL_BUFFER_TRANSPARENT);
     LoadWindowGfx(0, 0, 2, 224);
     LoadPalette(gUnknown_0860F074, 0xF0, 0x20);
 }