core: Update docs according to recent async changes
core: Update docs according to recent async changes

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