A "Documentation Descriptor" is a short XML file that describes to the CoreDoc tool how it should go about generating documentation for a project.
The following shows a simple example documentation descriptor that is used to document the Core.JS JavaScript API contained in the Core.js and Core.Web.js modules:
<coredoc> <title>CoreJS API Specification</title> <modules> <module>Core.js</module> <module>Core.Web.js</module> </modules> </coredoc>
The document is contained within <coredoc> tags.  And as one can likely guess, the <title> tag contains the title of the documentation.
A list of modules to document is contained in the <modules> tag, with each individual module specified in a <module> element.  The file names in <module> elements are relative to the path of the doc.xml descriptor file.  Optionally, a base attribute may be specified in the <modules> attribute to describe the directory in which all contained modules are located, e.g.:
<modules base="path/to/modules/">