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

usr.sbin/jail/jail.conf.5

@@ -22,7 +22,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd Jun 3, 2023
+.Dd September 5, 2023
 .Dt JAIL.CONF 5
 .Os
 .Sh NAME
@@ -164,11 +164,22 @@ and
 .Ss Includes
 A line of the form
 .Bd -literal -offset ident
-.include "filename";
+\&.include "filename";
 .Ed
 .Pp
-will include another file in the configuration.  The filename must be
-a literal string, and cannot contain variable expansions.
+will include another file (or files) in the configuration.
+The filename should be either absolute, or relative to the
+configuration file's directory.  It cannot contain variable
+expansions, but may contain
+.Xr glob 3
+patterns.
+.Pp
+The included file must exist, though a filename glob may match zero or
+more files.  This allows inclusion of any/all files in a directory,
+such as
+.Dq /etc/jail.conf.d/*.conf ,
+or conditional inclusion of a single file, such as
+.Dq jail.foo[.]conf .
 .Ss Comments
 The configuration file may contain comments in the common C, C++, and
 shell formats: