docs: mention extra input-selector pad properties
docs: mention extra input-selector pad properties

https://bugzilla.gnome.org/show_bug.cgi?id=638381

Distrotech/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,mrchapp/gstreamer,collects/gstreamer,mparis/gstreamer,cfoch/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,justinjoy/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,ahmedammar/platform_external_gst_gstreamer,magcius/gstreamer,surround-io/gstreamer,mparis/gstreamer,drothlis/gstreamer,mparis/gstreamer,collects/gstreamer,ahmedammar/platform_external_gst_gstreamer,mrchapp/gstreamer,mrchapp/gstreamer,ensonic/gstreamer,jpxiong/gstreamer,magcius/gstreamer,magcius/gstreamer,centricular/gstreamer,krieger-od/gstreamer,cablelabs/gstreamer,magcius/gstreamer,drothlis/gstreamer,krieger-od/gstreamer,ensonic/gstreamer,ylatuya/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,StreamUtils/gstreamer,ahmedammar/platform_external_gst_gstreamer,StreamUtils/gstreamer,mrchapp/gstreamer,surround-io/gstreamer,lubosz/gstreamer,krichter722/gstreamer,drothlis/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,Lachann/gstreamer,shelsonjava/gstreamer,ensonic/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,lubosz/gstreamer,collects/gstreamer,Distrotech/gstreamer,shelsonjava/gstreamer,Distrotech/gstreamer,magcius/gstreamer,Lachann/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,cfoch/gstreamer,StreamUtils/gstreamer,drothlis/gstreamer,lovebug356/gstreamer,Distrotech/gstreamer,ylatuya/gstreamer,justinjoy/gstreamer,mparis/gstreamer,ylatuya/gstreamer,justinjoy/gstreamer,drothlis/gstreamer,collects/gstreamer,collects/gstreamer,cablelabs/gstreamer,ahmedammar/platform_external_gst_gstreamer,lubosz/gstreamer,krichter722/gstreamer,lovebug356/gstreamer,ylatuya/gstreamer,centricular/gstreamer,centricular/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,centricular/gstreamer,surround-io/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,lubosz/gstreamer,cfoch/gstreamer,Lachann/gstreamer,surround-io/gstreamer,cfoch/gstreamer,ylatuya/gstreamer,jpxiong/gstreamer,justinjoy/gstreamer,StreamUtils/gstreamer,ahmedammar/platform_external_gst_gstreamer,krieger-od/gstreamer,ensonic/gstreamer,mrchapp/gstreamer,justinjoy/gstreamer,jpxiong/gstreamer,Distrotech/gstreamer,centricular/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer