Update for_expr docs.
auto merge of #9039 : singingboyo/rust/update-for-expr-docs, r=thestinger

The old documentation for for loops/expressions has been quite wrong since the change to iterators.  This updates the docs to make them relevant to how for loops work now, if not very in-depth.  There may be a need for updates giving more depth on how they work, such as detailing what method calls they make, but I don't know enough about the implementation to include that.