man mdadm does not document the --offroot option

I can see in the logs that systemd incrementally assembles mdadm arrays using the --incremental and --offroot options, yet the --offroot option is not documented in the mdadm manpage.

I can see it here though: https://linux.die.net/man/8/mdadm

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: mdadm 3.3-2ubuntu7.6
ProcVersionSignature: Ubuntu 4.15.0-72.81~16.04.1-generic 4.15.18
Uname: Linux 4.15.0-72-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.21
Architecture: amd64
Date: Sun Jan 5 09:07:20 2020
InstallationDate: Installed on 2018-04-27 (617 days ago)
InstallationMedia: Ubuntu 16.04.4 LTS "Xenial Xerus" - Release amd64 (20180228)
MDadmExamine.dev.sda: Error: command ['/sbin/mdadm', '-E', '/dev/sda'] failed with exit code 1: mdadm: cannot open /dev/sda: Permission denied
MDadmExamine.dev.sdb: Error: command ['/sbin/mdadm', '-E', '/dev/sdb'] failed with exit code 1: mdadm: cannot open /dev/sdb: Permission denied
MDadmExamine.dev.sdb1: Error: command ['/sbin/mdadm', '-E', '/dev/sdb1'] failed with exit code 1: mdadm: cannot open /dev/sdb1: Permission denied
MDadmExamine.dev.sdc: Error: command ['/sbin/mdadm', '-E', '/dev/sdc'] failed with exit code 1: mdadm: cannot open /dev/sdc: Permission denied
MDadmExamine.dev.sdc1: Error: command ['/sbin/mdadm', '-E', '/dev/sdc1'] failed with exit code 1: mdadm: cannot open /dev/sdc1: Permission denied
MDadmExamine.dev.sdd: Error: command ['/sbin/mdadm', '-E', '/dev/sdd'] failed with exit code 1: mdadm: cannot open /dev/sdd: Permission denied
MDadmExamine.dev.sde: Error: command ['/sbin/mdadm', '-E', '/dev/sde'] failed with exit code 1: mdadm: cannot open /dev/sde: Permission denied
MDadmExamine.dev.sde1: Error: command ['/sbin/mdadm', '-E', '/dev/sde1'] failed with exit code 1: mdadm: cannot open /dev/sde1: Permission denied
MDadmExamine.dev.sde2: Error: command ['/sbin/mdadm', '-E', '/dev/sde2'] failed with exit code 1: mdadm: cannot open /dev/sde2: Permission denied
MDadmExamine.dev.sdf: Error: command ['/sbin/mdadm', '-E', '/dev/sdf'] failed with exit code 1: mdadm: cannot open /dev/sdf: Permission denied
MDadmExamine.dev.sdg: Error: command ['/sbin/mdadm', '-E', '/dev/sdg'] failed with exit code 1: mdadm: cannot open /dev/sdg: Permission denied
MachineType: To Be Filled By O.E.M. To Be Filled By O.E.M.
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-72-generic root=UUID=2a39e062-c0b5-4227-90f3-8f0c9169986d ro quiet i915.alpha_support=1i fsck.repair=yes
SourcePackage: mdadm
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 03/27/2019
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: P4.00
dmi.board.name: Z370M-ITX/ac
dmi.board.vendor: ASRock
dmi.chassis.asset.tag: To Be Filled By O.E.M.
dmi.chassis.type: 3
dmi.chassis.vendor: To Be Filled By O.E.M.
dmi.chassis.version: To Be Filled By O.E.M.
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvrP4.00:bd03/27/2019:svnToBeFilledByO.E.M.:pnToBeFilledByO.E.M.:pvrToBeFilledByO.E.M.:rvnASRock:rnZ370M-ITX/ac:rvr:cvnToBeFilledByO.E.M.:ct3:cvrToBeFilledByO.E.M.:
dmi.product.family: To Be Filled By O.E.M.
dmi.product.name: To Be Filled By O.E.M.
dmi.product.version: To Be Filled By O.E.M.
dmi.sys.vendor: To Be Filled By O.E.M.
etc.blkid.tab: Error: [Errno 2] No such file or directory: '/etc/blkid.tab'