docs: RygelDataSource: Add description
docs: RygelDataSource: Add description

Add some text from Mark Ryan.

GNOME/rygel,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,jku/rygel,Pelagicore/Media-Manager-Rygel-patched,Pelagicore/Media-Manager-Rygel-patched,GNOME/rygel,GNOME/rygel,Tarnyko/rygel,Tarnyko/rygel