add doc comment for object-moved signal
add doc comment for object-moved signal

ahmedammar/platform_external_gst_editing_services,cfoch/ges,cfoch/gst-editing-services,freesteph/gst-editing-services,MathieuDuponchelle/PitiviGes,freesteph/gst-editing-services,MathieuDuponchelle/PitiviGes,cfoch/gst-editing-services,cfoch/gst-editing-services,volodymyrrudyi/gst-editing-services,cfoch/ges,volodymyrrudyi/gst-editing-services,volodymyrrudyi/gst-editing-services,cfoch/gst-editing-services,freesteph/gst-editing-services,cfoch/ges,ahmedammar/platform_external_gst_editing_services,MathieuDuponchelle/PitiviGes,luisbg/gst-editing-services,pitivi/gst-editing-services,pitivi/gst-editing-services,MathieuDuponchelle/PitiviGes,luisbg/gst-editing-services,pitivi/gst-editing-services,cfoch/ges,pitivi/gst-editing-services