MFC jail.conf(5): Fix and expand description of ".include".

Escape a period that caused a line not to be includesd. Also mention
that glob(3) patterns may be included, and a consequence of that.

PR: 273561
Reported by: crest@rlwinm.de
Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D41723

(cherry picked from commit 165234d5527440b7755b01554ff7633e33c0d285)