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

(cherry picked from commit 27130028a583273071a14a3256f8ea0a131f2dd4)