A Module which describes other modules
Now there is a module for that ;) It's the Developer Docs module.
Here's a brief description about it (from its project page):

Developer Docs is a module specially made for drupal developers (amateur). When a new module is installed, it is hard to find what changes it does unless we look into the code. This module provides an overview of &quot;what it does&quot; without looking into the code.
This Module generates a document automatically based on the drupal elements used in the module such as:

Administration menu
Hooks
Database Schemas
Themes
Variables
Blocks
Node types
Entity Types
Fields (Formatter, Widget)
Field Formatters
Field Widgets
Hooks Created

After enabling this module, list of installed modules will be available at admin/reports/developer-docs/