Document behavior of using iterator_to_array() with a ChangeStream

The example for change stream iteration relies on a manual iteration loop and does not stop iteration when valid() is false. This is standard practice for iterating a tailable cursor.

That said, some users may still end up using iterator_to_array() to collect events from a change stream. In that case, iteration will naturally stop when no more events are available. We should discuss this on the documentation for the watch() function, alongside any examples for manual iteration.