JT tech review 1

docs/eloquent-models/model-class.txt

@@ -297,8 +297,10 @@ MongoDB.
 When you apply the ``DocumentModel`` trait to a model class, you must
 declare the following properties in your class:
 
-- ``$primaryKey``: Defines the field that uniquely identifies a MongoDB document
-- ``$keyType``: Defines the type of the primary key value
+- ``$primaryKey = '_id'``, because the ``_id`` field uniquely
+  identifies MongoDB documents
+- ``$keyType = 'string'``, because {+odm-short+} casts MongoDB
+  ``ObjectId`` values to type ``string``
 
 Extended Class Example
 ~~~~~~~~~~~~~~~~~~~~~~