Remove reference to doc type in persistence docs

Closes #1204

Author: honzakral

docs/persistence.rst

@@ -394,10 +394,6 @@ the index, its name, settings and other attributes:
   used for any write operations and an ``index`` kwarg will have to be passed
   explicitly when calling methods like ``.save()``.
 
-``doc_type``
-  name of the ``_type`` in elasticsearch. Note that you have to define this as
-  well as ``doc_type`` in ``class Meta`` in order for it to take effect.
-
 ``using``
   default connection alias to use, defaults to ``'default'``