Background & Motivation
There have been several questions about how to iterate a change stream to block indefinitely until the next notification or error:
CXX-2435, CXX-2278, and HELP-28966. It is not clear from the documentation how to accomplish this.

Scope
Propose the following improvements to documentation:

1. Add a note in the function comments of change_stream::iterator::operator++
If no notification is available, callers may call change_stream::begin() to check for more notifications.

2. Add a note in the function comments of change_stream::iterator::operator* and change_stream::iterator::operator->:
The returned document::view is valid until the iterator is incremented. The value may be copied to extend its lifetime.

3. Update the example in https://github.com/mongodb/mongo-cxx-driver/blob/master/examples/mongocxx/change_streams.cpp. Change watch_until to resemble the following:

auto cs = db.collection("coll").watch(); // sends one aggregate command.
auto cs_iter = cs.begin(); // may send getMore if more events are needed. May send aggregate if resume is needed.
while (true) {
    if (cs_iter == cs.end()) {
        std::cout << "No event yet." << std::endl;
        cs_iter = cs.begin (); // may send getMore if more events are needed. May send aggregate if resume is needed.
    } else {
        bsoncxx::document::view event = *cs_iter;
        std::cout << "Got event: " << bsoncxx::to_json (event) << std::endl;
        cs_iter++; // may send getMore if more events are needed. May send aggregate if resume is needed.
    }
}

Waiting for a notification or error is the most common use case.