etc/kamailio.cfg: document tls_threads_mode = 2 to use atfork handler

etc/kamailio.cfg

@@ -224,8 +224,9 @@ tls_max_connections=2048
 /* For OpenSSL 3 integration
  * functions calling libssl3 can be invoked in a transient thread
  * 0: disable threaded calls
- * 1: use threads for process#0 only
- * 2: use threads for all processes */
+ * 1: use thread executors for process#0 only
+ * 2: no thread executors, but use atfork handler to reset thread-locals to NULL
+ * 3: use thread executors for all processes */
 tls_threads_mode=1
 #!endif