element: Document when a clock is available from gst_element_get_clock()
element: Document when a clock is available from gst_element_get_clock()

https://bugzilla.gnome.org/show_bug.cgi?id=744442

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