jail: document the mac.label parameter

In particular, we should provide a hint about mac.conf(5), since libjail
will just use the mac_prepare_type(3) API to provide a reasonably sane
interface for system administrators.  Progammers wanting to fetch an
arbitrary MAC label would need to bypass libjail and use jail_get(2)
directly with their own prepared `struct mac`.

Differential Revision:	https://reviews.freebsd.org/D54067

share/man/man4/mac.4

@@ -28,7 +28,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd December 30, 2024
+.Dd January 15, 2026
 .Dt MAC 4
 .Os
 .Sh NAME
@@ -165,6 +165,7 @@ and modifying its MAC policy label.
 .Bl -column "user (by login class)" "Xr setfmac 8 , Xr setfsmac 8" -offset indent
 .It Sy "Subject/Object" Ta Sy "Utility"
 .It "File system object" Ta Xr setfmac 8 , Xr setfsmac 8
+.It Jail Ta Xr jail 8
 .It "Network interface" Ta Xr ifconfig 8
 .It "TTY (by login class)" Ta Xr login.conf 5
 .It "User (by login class)" Ta Xr login.conf 5
@@ -221,6 +222,7 @@ man page.
 .Xr mac_test 4 ,
 .Xr login.conf 5 ,
 .Xr maclabel 7 ,
+.Xr jail 8 ,
 .Xr getfmac 8 ,
 .Xr getpmac 8 ,
 .Xr setfmac 8 ,

usr.sbin/jail/jail.8

@@ -26,7 +26,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd November 13, 2025
+.Dd December 4, 2025
 .Dt JAIL 8
 .Os
 .Sh NAME
@@ -503,6 +503,15 @@ pseudo-parameter set.
 The ID of the cpuset associated with this jail (read-only).
 .It Va dying
 This is true if the jail is in the process of shutting down (read-only).
+.It Va mac.label
+The
+.Xr mac 3
+label associated with this jail.
+Note that a
+.Dq jail
+entry in
+.Xr mac.conf 5
+may need to be configured in order to retrieve the MAC label.
 .It Va parent
 The
 .Va jid
@@ -1550,6 +1559,7 @@ environment of the first jail.
 .Xr clock_settime 2 ,
 .Xr jail_set 2 ,
 .Xr ntp_adjtime 2 ,
+.Xr mac 3 ,
 .Xr devfs 4 ,
 .Xr fdescfs 4 ,
 .Xr linprocfs 4 ,
@@ -1557,6 +1567,7 @@ environment of the first jail.
 .Xr procfs 4 ,
 .Xr vmm 4 ,
 .Xr jail.conf 5 ,
+.Xr mac.conf 5 ,
 .Xr rc.conf 5 ,
 .Xr sysctl.conf 5 ,
 .Xr bsdconfig 8 ,