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The two-arg type form is intended for use with classmethods,
e.g.::

>>> class C(object):
...     @classmethod
...     def f(cls):
...         print 'C.f'
...         
>>> class D(C):
...     @classmethod
...     def f(cls):
...         super(D, cls).f()
...         print 'D.f'
...         
>>> D.f()
C.f
D.f

I do make use of this occasionally, so I do think it should
be documented.

The one-arg form is intended to be used as a class attribute::

>>> class C(object):
...     def f(self):
...         print 'C.f'
... 
>>> class D(C):
...      def f(self):
...          self.super.f()
...          print 'D.f'
... 
>>> D.super = super(D)
>>> D().f()
C.f
D.f

I don't know that we should really be advertising this
one-arg form though.  It requires modifying a class outside
the definition or using some metaclass hackery to be usable.
 And it's misleading because it won't work, for example,
with classmethods:

>>> class C(object):
...     @classmethod
...     def f(cls):
...         print 'C.f'
... 
>>> class D(C):
...     @classmethod
...     def f(cls):
...         cls.super.f()
...         print 'D.f'
... 
>>> D.super = super(D)
>>> D().f()
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
  File "<interactive input>", line 4, in f
AttributeError: 'super' object has no attribute 'f'

That's why I tried to gloss over it lightly.  Personally I'd
prefer that the one-arg form was just deprecated.  It
introduces a lot more problems than it solves.

But if it has to be documented, it should probably say
something like "A super object created with a single
argument produces a descriptor object. This descriptor
object makes the superclass methods available on the object
returned by its __get__ method. The superclass methods are
only available when __get__ is called with an instance (not
a class)."