keyword in's doc uses says it requires an Iterator instead of an IntoIterator

[SkiFire13]

In the doc for the keyword in it's stated that "The expression immediately following in must implement the Iterator trait."
This is not completly right since it can be followed by anything that implements the IntoIterator trait.
The doc for the keyword for instead correctly cites the IntoIterator trait, althouth it could be improved since it just appears out of thin air when the desugared version is showed, without any prior mentioning.

This issue has been assigned to @Nicholas-Baron via this comment.

[Nicholas-Baron]

PR ready for review. I tried to keep the changes to a minimum. Hopefully, the wording is understandable.

I'll just assign myself.
@rustbot claim