[libc++][ranges][NFC] Update the status of predefined iterators.

`{back,front}_insert_iterator` and `ostream{,buf}_iterator` effectively
fully implement the One Ranges Proposal already, so mark them as done:
- the change to `difference_type` was made by D103273;
- default constructors and the associated default member initializers
  were removed by wg21.link/P2325 (implemented by D102468).

Also fix a stale template signature in the `<iterator>` synopsis.