Before we remove all this stuff in 4.0 we need to deprecate a few things that were not previously explicitly deprecated:

• The Database methods incoming_manipulators, incoming_copy_manipulators, outgoing_manipulators, outgoing_copy_manipulators
• The manipulate option for Cursor.
• pymongo/son_manipulator.py

Database.add_son_manipulator has raised DeprecationWarning since 3.0, so users already know manipulators are deprecated. The custom type docs also start with a large detailed warning. Let's just be consistent.