Update defining-documents.rst

Add primary_key notice according to issue #985

docs/guide/defining-documents.rst

@@ -146,6 +146,8 @@ arguments can be set on all fields:
     When True, use this field as a primary key for the collection.  `DictField`
     and `EmbeddedDocuments` both support being the primary key for a document.
 
+    .. note:: If set, this field is also accessible throught the `pk` field.
+
 :attr:`choices` (Default: None)
     An iterable (e.g. a list or tuple) of choices to which the value of this
     field should be limited.