Added documentation note for add_custom_type()

doc/classes/EditorPlugin.xml

@@ -409,6 +409,7 @@
 			<description>
 				Adds a custom type, which will appear in the list of nodes or resources. An icon can be optionally passed.
 				When a given node or resource is selected, the base type will be instantiated (e.g. "Node3D", "Control", "Resource"), then the script will be loaded and set to this object.
+				[b]Note:[/b] The base type is the base engine class which this type's class hierarchy inherits, not any custom type parent classes.
 				You can use the virtual method [method _handles] to check if your custom object is being edited by checking the script or using the [code]is[/code] keyword.
 				During run-time, this will be a simple object with a script so this function does not need to be called then.
 				[b]Note:[/b] Custom types added this way are not true classes. They are just a helper to create a node with specific script.