Check out our current Marble KDE handbook:

http://docs.kde.org/stable/en/kdeedu/marble/index.html

Create a chapter about the stars plugin and its capabilities.

For technical details about writing such documentation have a look at the source code (kdeedu/docs) and have a look at KDE's developer guides covering documentation, e.g. 

http://techbase.kde.org/Documentation_Primer/Manual/DocBook

The handbook should describe the user features and capabilities as well as the stars plugin settings dialog.