bpo-40636: Clarify the zip built-in docstring. (GH-20118)

bpo-40636: Clarify the zip built-in docstring. (GH-20118)

Clarify the zip built-in docstring.

This puts much simpler text up front along with an example.

As it was, the zip built-in docstring was technically correct.  But too
technical for the reader who shouldn't _need_ to know about `__next__` and
`StopIteration` as most people do not need to understand the internal
implementation details of the iterator protocol in their daily life.

This is a documentation only change, intended to be backported to 3.8; it is
only tangentially related to PEP-618 which might offer new behavior options
in the future.

Wording based a bit more on enumerate per Brandt's suggestion.

This gets rid of the legacy wording paragraph which seems too tied to
implementation details of the iterator protocol which isn't relevant here.

Co-authored-by: Brandt Bucher <brandtbucher@gmail.com>