libcamera: message: Document Message::SignalMessage

The SignalMessage type is undocumented. Fix it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/message.cpp

@@ -41,6 +41,8 @@ LOG_DEFINE_CATEGORY(Message)
  * \brief The message type
  * \var Message::None
  * \brief Invalid message type
+ * \var Message::SignalMessage
+ * \brief Asynchronous signal delivery across threads
  */
 
 /**