Add documentation

Closes #1004

av/container/core.pyx

@@ -365,6 +365,7 @@ def open(file, mode=None, format=None, options=None,
         ``url`` is the url to open, ``flags`` is a combination of AVIO_FLAG_* and
         ``options`` is a dictionary of additional options. The callable should return a
         file-like object.
+    :rtype: Container
 
     For devices (via ``libavdevice``), pass the name of the device to ``format``,
     e.g.::

av/container/output.pyx

@@ -53,6 +53,9 @@ cdef class OutputContainer(Container):
             Examples for video include ``24``, ``23.976``, and
             ``Fraction(30000,1001)``. Examples for audio include ``48000``
             and ``44100``.
+        :param template: Copy codec from another :class:`~av.stream.Stream` instance.
+        :param dict options: Stream options.
+        :param \\**kwargs: Set attributes of the stream.
         :returns: The new :class:`~av.stream.Stream`.
 
         """