Adding doc snippet for paging with cursors.

Author: dhermes

datastore/google/cloud/datastore/client.py

@@ -464,6 +464,23 @@ def query(self, **kwargs):
           >>> for entity in query_iter:
           ...     do_something(entity)
 
+        or manually page through results
+
+        .. code-block:: python
+
+          >>> query_iter = query.fetch(start_cursor='2mdd223i944')
+          >>> pages = query_iter.pages
+          >>>
+          >>> first_page = next(pages)
+          >>> first_page_entities = list(first_page)
+          >>> query_iter.next_page_token
+          'abc-some-cursor'
+          >>>
+          >>> second_page = next(pages)
+          >>> second_page_entities = list(second_page)
+          >>> query_iter.next_page_token is None
+          True
+
         Under the hood this is doing:
 
         .. code-block:: python