DOC: Clarify documentation of the extern keyword

src/doc/yaml_interface.rst

-
 ===============================
  YAML data model specification
 ===============================
@@ -70,9 +69,22 @@ Note the difference between the three property declarations of ``LabbookEntry``:
 - ``responsible``: This defines and adds a property with name "responsible" to ``LabbookEntry`, which has a datatype ``Person``. ``Person`` is defined above.
 - ``firstName``: This defines and adds a property with the standard data type ``TEXT`` to record type ``Person``.
 
-If the data model depends on record types or properties which already exist in CaosDB, those can be
-added using the ``extern`` keyword: ``extern`` takes a list of previously defined names.
+If the data model depends on record types or properties which already exist in
+LinkAhead, those can be added using the ``extern`` keyword: ``extern`` takes a
+list of previously defined names of Properties and/or RecordTypes. Note that if you happen to use an already existing ``REFERENCE`` property that has an already existing RecordType as datatype, you also need to add that RecordType's name to the ``extern`` list, e.g.,
+
+.. code-block:: yaml
+
+   extern:
+     # Let's assume the following is a reference property with datatype Person
+     - Author
+     # We need Person (since it's the datatype of Author) even though we might
+     # not use it explicitly
+     - Person
 
+   Dataset:
+     recommended_properties:
+       Author:
 
 
 Datatypes