Some moderate refactoring for boto3 docstrings

[kyleknap]

Includes extracting out the logic that creates the client classes into a public function. This is needed because resource factories in boto3 do not have access to the client's class, and thus its name, when creating the resource class.

There was also some moving around some of the internals of the docstring class to make it easier to inherit from for example:

class ActionDocstring(LazyLoadedDocstring):
    def __init__(self, *args, **kwargs):
        super(ActionDocstring, self).__init__(*args, **kwargs)
        self._docstring_writer = document_action

cc @jamesls @mtdowling @rayluo

[mtdowling]

🚢

[rayluo]

🚢