doc: add GST_{PTR,SEGMENT}_FORMAT
doc: add GST_{PTR,SEGMENT}_FORMAT

Very useful formats in debug output so best to have them in the
generated doc.

https://bugzilla.gnome.org/show_bug.cgi?id=756115

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