wayland-server: hide wl_priv_signal from doxygen

Fix this set of warnings appearing three times during a build:

/home/pq/git/wayland/src/wayland-server.c:1868: warning: class
`wl_priv_signal' for related function `wl_priv_signal_init' is not
documented.
/home/pq/git/wayland/src/wayland-server.c:1884: warning: class
`wl_priv_signal' for related function `wl_priv_signal_add' is not
documented.
/home/pq/git/wayland/src/wayland-server.c:1899: warning: class
`wl_priv_signal' for related function `wl_priv_signal_get' is not
documented.

Our Wayland docbook don't include private things, so make sure these do
not end up there. This removes the mention of wl_priv_signal_emit from
the Server API docbook. I have no idea why the other functions did not
appear there.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
diff --git a/src/wayland-server.c b/src/wayland-server.c
index cdd46fa..6a8b3e4 100644
--- a/src/wayland-server.c
+++ b/src/wayland-server.c
@@ -1881,6 +1881,8 @@
 	wl_map_for_each(&client->objects, resource_iterator_helper, &context);
 }
 
+/** \cond INTERNAL */
+
 /** Initialize a wl_priv_signal object
  *
  * wl_priv_signal is a safer implementation of a signal type, with the same API
@@ -1972,6 +1974,8 @@
 	}
 }
 
+/** \endcond INTERNAL */
+
 /** \cond */ /* Deprecated functions below. */
 
 uint32_t