controlsource: fix GetValue and GetValueArray documentation
controlsource: fix GetValue and GetValueArray documentation

GstControlSourceGetValue() value paramater is a gdouble, not a GValue
and GstControlSourceGetValueArray doesn't return a GstValueArray but
an array of double.

https://bugzilla.gnome.org/show_bug.cgi?id=758668

StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer