clock: Add new signed time macro to the doc
clock: Add new signed time macro to the doc

https://bugzilla.gnome.org/show_bug.cgi?id=740575

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