server: document protocol logger callback function

This fixes a validation error in the documentation which are not fatal.

The idea is the same as in 12ec67a ("server: document listener fields
and a vfunc"), which seems to have missed the reference to the
wl_protocol_logger_func_t from the note in wl_log_func_t.

Signed-off-by: Sebastian Wick <sebastian.wick@redhat.com>

src/wayland-server-core.h

@@ -713,6 +713,7 @@ struct wl_protocol_logger_message {
 	const union wl_argument *arguments;
 };
 
+/** Callback function type for wl_display_add_protocol_logger() */
 typedef void (*wl_protocol_logger_func_t)(void *user_data,
 					  enum wl_protocol_logger_type direction,
 					  const struct wl_protocol_logger_message *message);