b2: add note about cleanup in docs
b2: add note about cleanup in docs

Added: "Note that `cleanup` does not remove partially uploaded filesfrom the bucket."
B4dM4n/rclone,B4dM4n/rclone,ncw/rclone,ncw/rclone,olihey/rclone,ncw/rclone,olihey/rclone,olihey/rclone,B4dM4n/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone