[issue20823] Clarify copyreg.pickle() documentation

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The phrase "*function* should return either a string or a tuple containing two or three elements." de-facto is outdated too. Since Python 2.3 a tuple returned by the function must have two to five elements.

copyreg.constructor() no longer does anything useful, and the constructor parameter of copyreg.pickle() is virtually ignored.

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nosy: +serhiy.storchaka
stage:  -> patch review
type:  -> behavior
versions: +Python 3.5, Python 3.6, Python 3.7 -Python 3.3

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