ktls.4: Mention mb_use_ext_pgs in the "sysctl nodes" section

Reviewed by:	rmacklem, jhb
MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D30403

share/man/man4/ktls.4

@@ -31,7 +31,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd May 20, 2021
+.Dd May 26, 2021
 .Dt KTLS 4
 .Os
 .Sh NAME
@@ -199,6 +199,11 @@ A tree of nodes containing statistics for TLS sessions using
 .It Va kern.ipc.tls.stats
 A tree of nodes containing various kernel TLS statistics.
 .El
+.Pp
+The
+.Va kern.ipc.mb_use_ext_pgs
+sysctl controls whether the kernel may use unmapped mbufs.
+They are required for TLS transmit.
 .Ss Backends
 The base system includes a software backend for the
 .Dv TCP_TLS_MODE_SW