[docs] [issue14129] Corrections for the "extending" doc

[Eli Bendersky]

Eli Bendersky <eliben at gmail.com> added the comment:

"A PyObject is not a very magnificent object - it just contains the refcount and a pointer to the object’s “type object”."

Too chatty and should be replaced by a more pragmatic explanation, or shortened.

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nosy: +docs at python

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue14129>
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