CLEANUP: listener: add a comment about what the BC_SSL_O_* flags are for

They're for ->ssl_options but it wasn't obvious.
diff --git a/include/haproxy/listener-t.h b/include/haproxy/listener-t.h
index 5d2093a..3f0ea58 100644
--- a/include/haproxy/listener-t.h
+++ b/include/haproxy/listener-t.h
@@ -113,6 +113,7 @@
  * maxconn setting to the global.maxsock value so that its resources are reserved.
  */
 
+/* flags used with bind_conf->ssl_options */
 #ifdef USE_OPENSSL
 #define BC_SSL_O_NONE           0x0000
 #define BC_SSL_O_NO_TLS_TICKETS 0x0100	/* disable session resumption tickets */