End-user Documentation Generation
Drupal has an API for documentation and comment standards that allow you embed comments in the code using a special syntax that will automatically generate documentation for the module, including modules that create content types.

This is the system that is used to automatically generate the online API reference  found on http://api.drupal.org and other similar sites.

The API module used to generate these pages parses documentation and code in PHP files, and it expects documentation to be in a format similar to other code/documentation parsing systems such as PHPDoc, JavaDoc, etc. The system was originally based on doxygen, but it has evolved into something that has its own set of tags and Drupal-specific functionality.