IntoIterator docs should show its usage as a bound

[steveklabnik]

IntoIterator has a complex set of associated types, and is commonly used as a bound on functions, but does not show its usage at all.

I'd be happy to help anyone out to make this better! Fundamentally, we should show a function that takes a generic argument that has a bound with IntoIterator.

[iwillspeak]

I'd like to have a go at this if that's OK. I'll open a pull request.

[peter-b]

@steveklabnik @alexcrichton Should this be closed now that #44668 has been merged?

[alexcrichton]

Indeed!