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

[STINNER Victor]

STINNER Victor added the comment:

str[a:b] returns a substring (characters), not an array of code points (numbers).

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nosy: +haypo

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21667>
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