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/pokeblock_feed.c

@@ -704,7 +704,7 @@ static void HandleInitWindows(void)
     DeactivateAllTextPrinters();
     LoadUserWindowBorderGfx(0, 1, 0xE0);
     LoadPalette(gUnknown_0860F074, 0xF0, 0x20);
-    FillWindowPixelBuffer(0, 0);
+    FillWindowPixelBuffer(0, PIXEL_BUFFER_TRANSPARENT);
     PutWindowTilemap(0);
     schedule_bg_copy_tilemap_to_vram(0);
 }