[docs] [issue21667] Clarify status of O(1) indexing semantics of str objects

[Nick Coghlan]

Nick Coghlan added the comment:

Saying that ord() and chr() switch between characters and code points is just plain wrong, since characters may be represented as multiple code points.

We may also want to explicitly note that the Unicode normalisation is implementation dependendent, and that CPython doesn't normalise implicitly except where specifically noted (i.e. during lexical analysis).

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<http://bugs.python.org/issue21667>
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