After the changes in PYTHON-1579 PyMongo 4.0 no longer decodes plus signs to spaces. We should raise a Warning when username/password includes a plus sign "+" to help users in migrating from 3.13 to 4.0

Another interesting point is that we should update the 3.13 documentation to tell users to user quote() instead of quote_plus. Using quote() is compatible with both PyMongo 3 and 4 because it works with both unquote_plus() and unquote().