Invalid option (-p), and man page for passwd does not contain -p option

When unlocking a user account (which doesn't have a password) with 'passwd' I get the message:

  passwd: unlocking the password would result in a passwordless account.
  You should set a password with usermod -p to unlock the password of this account.

The Man page for passwd does not describe the '-p' option.

If I try to use -p as an option,I get an unknown option error:

  passwd: invalid option -- 'p'
  Usage: passwd [options] [LOGIN]
  ...

This is in Ubuntu 16.04, source: shadow

Package: passwd
Priority: required
Section: admin
Installed-Size: 2363
Maintainer: Ubuntu Developers <email address hidden>
Original-Maintainer: Shadow package maintainers <email address hidden>
Architecture: amd64
Source: shadow
Version: 1:4.2-3.1ubuntu5
Replaces: manpages-tr (<< 1.0.5), manpages-zh (<< 1.5.1-1)
Depends: libaudit1 (>= 1:2.2.1), libc6 (>= 2.14), libpam0g (>= 0.99.7.1), libselinux1 (>= 1.32), libsemanage1 (>= 2.0.3), lsb-base (>= 4.1+Debian11ubuntu7), libpam-modules, debianutils (>= 2.15.2)
Filename: pool/main/s/shadow/passwd_4.2-3.1ubuntu5_amd64.deb
Size: 779644
MD5sum: d63ef1119cfdf76850618732a0320841
SHA1: 6a53cced36ce2ef45f1ed5c23321736f1d5adcfd
SHA256: 8f1cf17e3d04044156d892db3dddb97cd6753c1d7d74aadd11ba2ddfd5ee2592
Description-en_AU: change and administer password and group data
 This package includes passwd, chsh, chfn, and many other programs to
 maintain password and group data.
 .
 Shadow passwords are supported. See /usr/share/doc/passwd/README.Debian
Description-md5: 5bbd70e421ed3367a8299e53bd7afed4
Multi-Arch: foreign
Homepage: http://pkg-shadow.alioth.debian.org/
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu
Supported: 5y
Task: minimal