[docs] [issue19548] 'codecs' module docs improvements

[Zoinkity .]

Zoinkity . added the comment:

One glaring omission is any information about multibyte codecs--the class, its methods, and how to even define one.  

Also, the primary use for codecs.register would be to append a single codec to the lookup registry.  Simple usage of the method only provides lookup for the provided codecs and will not include regularly-accessible ones such as "utf-8".  It would be enormously helpful to provide an example of proper, safe usage.

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nosy: +Zoinkity..

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19548>
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