Clarify that EditorScript can only run using the internal script editor (#8623)

* Add note that _run() can only be called from Godot's internal script editor

Author: AdriaandeJongh

tutorials/plugins/running_code_in_the_editor.rst

@@ -509,6 +509,11 @@ currently focused on the script editor.
 
 Scripts that extend EditorScript must be ``@tool`` scripts to function.
 
+.. note::
+
+    EditorScripts can only be run from the Godot script editor. If you are using
+    an external editor, open the script inside the Godot script editor to run it.
+
 .. danger::
 
     EditorScripts have no undo/redo functionality, so **make sure to save your