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
+          >>>
+          >>> page1 = next(pages)
+          >>> entities1 = list(page1)
+          >>> query_iter.next_page_token
+          'abc-some-cursor'
+          >>>
+          >>> page2 = next(pages)
+          >>> entities2 = list(page2)
+          >>> query_iter.next_page_token is None
+          True
+
         Under the hood this is doing:
 
         .. code-block:: python