docs: expose GstPushSrcClass in documentation
docs: expose GstPushSrcClass in documentation

Might come in handy in case someone wants to derive from it.

surround-io/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,krichter722/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,mparis/gstreamer,Lachann/gstreamer,centricular/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,mparis/gstreamer,StreamUtils/gstreamer,jpakkane/gstreamer,centricular/gstreamer,Lachann/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,jpakkane/gstreamer,shelsonjava/gstreamer,shelsonjava/gstreamer,mparis/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,jpakkane/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,mparis/gstreamer,jpakkane/gstreamer,surround-io/gstreamer,surround-io/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,centricular/gstreamer,mparis/gstreamer,Lachann/gstreamer