doxygen: enable AUTOBRIEF, drop \brief

Especially for short descriptions, it is annoying to have to type \brief for
every single API doc.

Drop all \brief and enable the AUTOBRIEF feature of doxygen, which always takes
the first sentence of an API doc as the brief description.

Change-Id: I11a8a821b065a128108641a2a63fb5a2b1916e87
diff --git a/src/signal.c b/src/signal.c
index 1d11617..301b021 100644
--- a/src/signal.c
+++ b/src/signal.c
@@ -27,7 +27,7 @@
 
 /*! \addtogroup signal
  *  @{
- *  \brief Generic signalling/notification infrastructure
+ *  Generic signalling/notification infrastructure
  */
 
 /*! \file signal.c */
@@ -44,7 +44,7 @@
 };
 
 
-/*! \brief Register a new signal handler
+/*! Register a new signal handler
  *  \param[in] subsys Subsystem number
  *  \param[in] cbfn Callback function
  *  \param[in] data Data passed through to callback
@@ -72,7 +72,7 @@
 	return 0;
 }
 
-/*! \brief Unregister signal handler
+/*! Unregister signal handler
  *  \param[in] subsys Subsystem number
  *  \param[in] cbfn Callback function
  *  \param[in] data Data passed through to callback
@@ -92,7 +92,7 @@
 	}
 }
 
-/*! \brief dispatch (deliver) a new signal to all registered handlers
+/*! dispatch (deliver) a new signal to all registered handlers
  *  \param[in] subsys Subsystem number
  *  \param[in] signal Signal number,
  *  \param[in] signal_data Data to be passed along to handlers