Document Iterator::take method behaviour

[AngelicosPhosphoros]

Also adds guarantee that fn take() doesn't take more than n elements.

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[rustbot]

⚠️ Warning ⚠️

• This PR is based on an upstream commit that is 15 days old.

  It's recommended to update your branch according to the rustc-dev-guide.

[workingjubilee]

@the8472 Is there some obscure specialization we have that depends on not holding this guarantee?

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[the8472]

It's not true in general, even without specialization. Take implements DoubleEndedIterator and next_back will have to go over all the excess elements before yielding an element.