You can find information about the document manager in the following help article Document Manager
Here is some additional information on how the DocumentsPaths
By default, all the libraries from the current site are included in the DocumentsPaths
When you set path starting without slash (e.g. "DocumentLibrary"), you will refer a local library - library from the current Web. If you set path staring with slash (e.g. /ImagesLibrary), you will refer a library from the SiteCollection (root).
Lets open the SubSite1 ConfigFile.xml and set the following images path:
The DocumentManager will load the SiteCollection/SubSite1
If you set the path with slash the ImageManager will open the SiteCollection
So in your case you need to set all the libraries from your sub site in the ConfigFile.xml or ListConfigFile.xml, which are located in the /Program Files/Common Files/Microsoft Shared/web server extensions/wpresources/RadEditorSharePoint/5.x.x.0__1f131a624888eeed/Resources/ folder.
I hope this helps.
the Telerik team