Adds documentation comments to CoglGstVideoSink and makes it generate
a separate manual to contain it.
One thing that I wasn't able to figure out with this was how to get
the documentation to have correct references to the main Cogl docs.
You can pass arguments to gtkdoc-fixxref to point to other manuals,
but presumably this needs the installed locations and when the
Cogl-Gst documentation is generated the Cogl docs may not have been
installed yet.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 5acdf8db47311893a9cf9ea04a66a287b657b8b0)
