Improve help message for CLI verbs

[jacobperron]

E.g. when running ros2 bag -h.

Before:

usage: ros2 bag [-h] Call `ros2 bag <command> -h` for more detailed usage. ...

Various rosbag related sub-commands

optional arguments:
  -h, --help            show this help message and exit

Commands:
  info    ros2 bag info
  play    ros2 bag play
  record  ros2 bag record

  Call `ros2 bag <command> -h` for more detailed usage.

After:

usage: ros2 bag [-h] Call `ros2 bag <command> -h` for more detailed usage. ...

Various rosbag related sub-commands

optional arguments:
  -h, --help            show this help message and exit

Commands:
  info    Print info about a bag to the screen
  play    Play back ROS data from a bag
  record  Record ROS data to a bag

  Call `ros2 bag <command> -h` for more detailed usage.