BTreeMap/Set should specify iteration is ordered

[Roxxik]

I just wanted to note that it is nowhere mentioned that all iterators from there are in sorted order. While I have to admit that it makes sense for a ordered container to provide ordered iterators, it is not documented. In the examples it is assumed that the iterators are ordered, but nowhere explictly stated.
Neither in the BTreeMap nor the BTreeSet.

[steveklabnik]

part of #29348

[jonhoo]

This is not actually closed, as the BTreeSet documentation for iter() still does not mention this. The change was made only for BTreeMap.