Update comments on internal descriptor classes.
Fix docstring for internal Async descriptor.
DelegateMethod should inherit from ReadOnlyProperty instead of being it.
Extra type checking in _MotorBaseCursor().
Implement MotorGridFS.find, MotorGridOut.readchunk, and MotorGridOutCursor.
Synchrotests for GridFS.find and GridOutCursor.
Rename UnwrapAsync descriptor to more accurate "Unwrap".
Use more descriptive DelegateMethod where appropriate, although it's the same as ReadOnlyProperty.