Tag.__call__ docstring talks about registering flattens and flatteners

[exarkun]

Twice it talks about some sort of registration mechanism:

        Children may be other tag instances, strings, functions, or any other
        object which has a registered flatten.

        Attributes may be 'transparent' tag instances (so that
        C{a(href=transparent(data="foo", render=myhrefrenderer))} works),
        strings, functions, or any other object which has a registered
        flattener.

I think this code should be talking about objects that provide IRenderable instead.