Documentation patch attached against py3k branch.

Changes are:

* Added to documentation of JSONDecoder:

If *strict* is ``False`` (``True`` is the default), then control characters will be allowed inside strings.  Control characters in this context are those with character codes in the 0-31 range, including ``'\t'`` (tab), ``'\n'``, ``'\r'`` and ``'\0'``.

* Added clarification in documentation of json.load and json.dump that unless the cls kwarg is specified, the JSONEncoder/JSONDecoder class will be used.

* Mirrored these additions in the relevant doc-strings (JSONDecoder.__init__, json.load, json.loads, json.dump, json.dumps).

* Copied description of the object_pairs_hook kwargs from the documentation to the relevant doc-strings, which otherwise fully mirrored the documentation. (json.load, json.loads, JSONDecoder.__init__)