docs: clarify that return value of gst_filename_to_uri() must be freed
docs: clarify that return value of gst_filename_to_uri() must be freed

https://bugzilla.gnome.org/show_bug.cgi?id=747104

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