MFC zfs/jail: Document the zfs.mount_snapshot parameter in jail(8).

PR:		274263
Differential Revision:	https://reviews.freebsd.org/D45647

(cherry picked from commit 63875db56b)

sys/contrib/openzfs/module/os/freebsd/zfs/zfs_vfsops.c

@@ -100,7 +100,7 @@ static struct zfs_jailparam zfs_jailparam0 = {
 static int zfs_jailparam_slot;
 
 SYSCTL_JAIL_PARAM_SYS_NODE(zfs, CTLFLAG_RW, "Jail ZFS parameters");
-SYSCTL_JAIL_PARAM(_zfs, mount_snapshot, CTLTYPE_INT | CTLFLAG_RW, "I",
+SYSCTL_JAIL_PARAM(_zfs, mount_snapshot, CTLTYPE_INT | CTLFLAG_RW, "B",
 	"Allow mounting snapshots in the .zfs directory for unjailed datasets");
 
 SYSCTL_NODE(_vfs_zfs, OID_AUTO, version, CTLFLAG_RD, 0, "ZFS versions");

usr.sbin/jail/jail.8

@@ -23,7 +23,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd August 26, 2023
+.Dd June 24, 2024
 .Dt JAIL 8
 .Os
 .Sh NAME
@@ -761,7 +761,15 @@ the jail cannot perform any sysvmsg-related system calls.
 .It Va sysvsem, sysvshm
 Allow access to SYSV IPC semaphore and shared memory primitives, in the
 same manner as
-.Va sysvmsg.
+.Va sysvmsg .
+.It Va zfs.mount_snapshot
+Allow jailed users to access the contents of ZFS snapshots under the
+filesystem's
+.Pa .zfs
+directory.
+If
+.Va allow.mount.zfs
+is set, the snapshots may also be mounted.
 .El
 .Pp
 There are pseudo-parameters that are not passed to the kernel, but are