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/include/osmocom/core/signal.h b/include/osmocom/core/signal.h
index 5ed4e15..d7132f9 100644
--- a/include/osmocom/core/signal.h
+++ b/include/osmocom/core/signal.h
@@ -13,7 +13,7 @@
 #define OSMO_SIGNAL_SS_APPS		0
 #define OSMO_SIGNAL_SS_RESERVED		2147483648u
 
-/*! \brief signal subsystems */
+/*! signal subsystems */
 enum {
 	SS_L_GLOBAL		= OSMO_SIGNAL_SS_RESERVED,
 	SS_L_INPUT,
@@ -25,7 +25,7 @@
 #define OSMO_SIGNAL_T_APPS		0
 #define OSMO_SIGNAL_T_RESERVED		2147483648u
 
-/*! \brief signal types. */
+/*! signal types. */
 enum {
 	S_L_GLOBAL_SHUTDOWN	= OSMO_SIGNAL_T_RESERVED,
 };