Help people create MobPath as a child of Main

Better steer people to create MobPath as a child of Main, so that they
don't accidentally create it as a child of StartPosition (the previously
selected item), which has a transform.

If they happen to re-parent to Main after creating the Path2D as a child
of StartPosition, they end up with a transform on the Path2D, which
breaks the expected spawn point of mobs.

See https://github.com/godotengine/godot-docs/pull/10698#issuecomment-2664002759.

Closes #6372, closes #10698.

getting_started/first_2d_game/05.the_main_game_scene.rst

@@ -38,9 +38,9 @@ Spawning mobs
 -------------
 
 The Main node will be spawning new mobs, and we want them to appear at a random
-location on the edge of the screen. Add a :ref:`Path2D <class_Path2D>` node
-named ``MobPath`` as a child of ``Main``. When you select ``Path2D``, you will
-see some new buttons at the top of the editor:
+location on the edge of the screen. Click the ``Main`` node in the Scene dock, then
+add a child :ref:`Path2D <class_Path2D>` node named ``MobPath``. When you select
+``Path2D``, you will see some new buttons at the top of the editor:
 
 .. image:: img/path2d_buttons.webp