Fix description of debug.obsolete_panic.

MFC after:	1 week

sys/kern/subr_bus.c

@@ -5922,8 +5922,9 @@ devctl2_init(void)
  */
 static int obsolete_panic = 0;
 SYSCTL_INT(_debug, OID_AUTO, obsolete_panic, CTLFLAG_RWTUN, &obsolete_panic, 0,
-    "Bus debug level");
-/* 0 - don't panic, 1 - panic if already obsolete, 2 - panic if deprecated */
+    "Panic when obsolete features are used (0 = never, 1 = if osbolete, "
+    "2 = if deprecated)");
+
 static void
 gone_panic(int major, int running, const char *msg)
 {