Document how labels can be given to partitions.

mkimg.1

@@ -135,6 +135,12 @@ To do this, run the
 utility as follows:
 .Dl % mkimg -s mbr -b /boot/mbr -p freebsd:-'mkimg -s bsd -b /boot/boot \
 -p freebsd-ufs:=root-file-system.ufs -p freebsd-swap::1G' -o mbr-bsd.img
+.Pp
+For partitioning schemes that feature partition labels, the
+.Nm
+utility supports assigning labels to the partitions specified.
+In the following example the file system partition is labeled as 'backup':
+.Dl % mkimg -s gpt -p freebsd-ufs/backup:=file-system.ufs -o gpt.img
 .Sh SEE ALSO
 .Xr gpart 8
 .Xr makefs 8