man: update kzonesign

doc/man/kzonesign.1in

@@ -36,7 +36,10 @@ level margin: \\n[rst2man-indent\\n[rst2man-indent-level]]
 .SH DESCRIPTION
 .sp
 This utility reads the zone\(aqs zone file, signs the zone according to given
-configuration, and writes the signed zone file back.
+configuration, and writes the signed zone file back. An alternative mode
+is DNSSEC validation of the given zone. The signing or validation
+can run in parallel if enabled in the configuration (see policy.signing\-threads
+and zone.adjust\-threads).
 .SS Config options
 .INDENT 0.0
 .TP