Documented a limitation of Options.required_db_features.

docs/ref/models/options.txt

@@ -308,7 +308,8 @@ Django quotes column and table names behind the scenes.
     the model is considered during the migration phase. For example, if you set
     this list to ``['gis_enabled']``, the model will only be synchronized on
     GIS-enabled databases. It's also useful to skip some models when testing
-    with several database backends.
+    with several database backends. Avoid relations between models that may or
+    may not be created as the ORM doesn't handle this.
 
 ``required_db_vendor``
 ----------------------