Drupal - How to show available drush commands per module with a drush command?

You can use the --filter feature of Drush. For example, you can use drush --filter=features to get all the commands available from the Features module, or drush --filter=views to get all the Drush commands available from the Views module.

You could then also pipe that output into a file if you wanted (e.g. drush --filter=views > views_drush_commands.txt).

You can find the docs on the Drush homepage.

If the module have less than 4 commands you can't use the drush --filter=MODULE_NAME because you will obtain the following message:

[error]  The specified command category onlyone does not exist. 

In this case you can use drush --filter and select the number that correspond with Other commands (usually at the end of the list).

You can read more about this behavior in: How to integrate the drush help --filter option with your custom module?