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

[Antoine Pitrou]

Antoine Pitrou added the comment:

Two things:
- I don't think it's very helpful to use the term "code point" without explaining or introducing it ("character" at least can be understood intuitively)
- The mention of slicing is ambiguous: is slicing suppoded to be O(1)? how is indexing related to slicing?

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nosy: +pitrou

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