pad: Clarify pad probe return type documentation
pad: Clarify pad probe return type documentation

shelsonjava/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,centricular/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,krichter722/gstreamer,jpxiong/gstreamer