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

[Eli Bendersky]

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

Adding the documentation experts. 

I plan to apply a fix for these soon. If you guys have any objections, let me know.

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nosy: +eric.araujo, ezio.melotti, georg.brandl

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<http://bugs.python.org/issue14129>
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