update `pull` method docs

Signed-off-by: aiordache <anca.iordache@docker.com>

docker/api/image.py

@@ -349,7 +349,8 @@ class ImageApiMixin(object):
 
         Args:
             repository (str): The repository to pull
-            tag (str): The tag to pull
+            tag (str): The tag to pull. If ``tag`` is ``None`` or empty, it
+                is set to ``latest``.
             stream (bool): Stream the output as a generator. Make sure to
                 consume the generator, otherwise pull might get cancelled.
             auth_config (dict): Override the credentials that are found in the
@@ -358,7 +359,8 @@ class ImageApiMixin(object):
             decode (bool): Decode the JSON data from the server into dicts.
                 Only applies with ``stream=True``
             platform (str): Platform in the format ``os[/arch[/variant]]``
-            all_tags (bool): Pull all image tags.
+            all_tags (bool): Pull all image tags, the ``tag`` parameter is
+                ignored.
 
         Returns:
             (generator or str): The output

docker/models/images.py

@@ -399,6 +399,7 @@ class ImageCollection(Collection):
         """
         Pull an image of the given name and return it. Similar to the
         ``docker pull`` command.
+        If ``tag`` is ``None`` or empty, it is set to ``latest``.
         If ``all_tags`` is set, the ``tag`` parameter is ignored and all image
         tags will be pulled.
 
@@ -417,7 +418,7 @@ class ImageCollection(Collection):
 
         Returns:
             (:py:class:`Image` or list): The image that has been pulled.
-                If ``tag`` is None, the method will return a list
+                If ``all_tags`` is True, the method will return a list
                 of :py:class:`Image` objects belonging to this repository.
 
         Raises: