DOC: halog: explain how to use -ac and -ad in the help message

Tim reported in issue #1435 that halog options -ac/-ad were poorly
documented. They're indeed used to spot infrastructure outages between
the clients and haproxy by detecting abnormal periods of silence followed
by bursts, either affecting the network itself, or also a single machine
(e.g. swapping on an edge client or proxy can cause such patterns).

admin/halog/halog.c

@@ -207,6 +207,11 @@ void help()
 	       "       -uao, -uto: average times computed on valid ('OK') requests\n"
 	       "       -uba, -ubt: average bytes returned, total bytes returned\n"
 	       " -hdr  output captured header at the given <block>:<field>\n",
+	       " -ac <count> -ad <delay>:\n
+	       "       Report periods corresponding to a grouped accept of <count> requests at\n"
+	       "       the same millisecond after a delay of at least <ad> milliseconds with no\n"
+	       "       incoming accept (used to spot network outages). Output format contains:\n"
+	       "          <accept_date> <date_ms> <delta_ms from previous one> <nb_entries>\n"
 	       (int)SOURCE_FIELD, (int)SOURCE_FIELD
 	       );
 	exit(0);