Explain how to setup GUI in 3D (#1149)

Make it more obvious to newcomers how this demo works so they can
replicate it in their projects.

Change the text to point to how this is working.

Add Editor Descriptions to make it clearer how this is setup.

Co-authored-by: Aaron Franke <arnfranke@yahoo.com>

viewport/gui_in_3d/gui_panel_3d.tscn

@@ -23,6 +23,7 @@ process_mode = 3
 script = ExtResource("1")
 
 [node name="SubViewport" type="SubViewport" parent="."]
+editor_description = "SubViewport contents can be displayed using a ViewportTexture."
 gui_embed_subwindows = true
 size = Vector2i(560, 360)
 render_target_update_mode = 4
@@ -51,9 +52,11 @@ grow_vertical = 2
 theme_override_constants/separation = 13
 
 [node name="Label" type="Label" parent="SubViewport/GUI/Panel/VBoxContainer"]
+custom_minimum_size = Vector2(200, 0)
 layout_mode = 2
-text = "Hello world!"
+text = "SubViewport is rendered on Quad"
 horizontal_alignment = 1
+autowrap_mode = 2
 
 [node name="Button" type="Button" parent="SubViewport/GUI/Panel/VBoxContainer"]
 layout_mode = 2
@@ -129,6 +132,12 @@ popup/item_2/text = "Item 2"
 popup/item_2/id = 2
 
 [node name="Quad" type="MeshInstance3D" parent="."]
+editor_description = "To setup this MeshInstance3D:
+* Set Surface Material Override to a new StandardMaterial3D.
+* At the bottom of the StandardMaterial3D's properties, set \"Local to Scene\" to true.
+* Set the Albedo Texture of StandardMaterial3D to new ViewportTexture and set the Viewport Path.
+* Transparency to Alpha.
+* Shading to Unshaded."
 mesh = SubResource("1")
 surface_material_override/0 = SubResource("3")