Add doctests for `iter_mut` and `into_iter` of BTreeMap.

Add spacing as dictated by standard rust code style.
Remove references to traits that no longer exist

This specifically means:
- `Deque`
- `Map`
- `Set`
auto merge of #19671 : tbu-/rust/pr_doc_removetraitrefs, r=Gankro

This specifically means:
- `Deque`
- `Map`
- `Set`
auto merge of #19685 : jbranchaud/rust/add-btreemap-iter-doctests, r=Gankro