[docs] [issue22725] improve documentation for enumerate() (built-in function)

[Van Ly]

Van Ly added the comment:

> "sequence must be a sequence"

The subtle minutiae of aficionados necessary to interpret the meaning of those two words in their distinct relation is opaque to a new comer and doesn’t serve the widest possible audience usefully to get the job done quick. The second placed sense has a range of possibility narrower than iterable as has been said, all sequences are iterables but not all iterables sequences.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue22725>
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