Many of motor's API method docstrings are duplicated in the doc/ folder. Many of these are out of date. We should remove the duplication so that the docs always match the docstrings in the source code.

For example the MotorGridFSBucket __init__ documentation is duplicated here:

• https://github.com/mongodb/motor/blob/2.3.1/doc/api-tornado/gridfs.rst#L14
• https://github.com/mongodb/motor/blob/2.3.1/motor/motor_gridfs.py#L341