[docs]: Explicitly mention the need to put `- default` in the users array if we want to modify an existing user

[mostafaCamel]

Documentation request

While creating a VM in openstack with the following userdata

users:
  - name: ubuntu    
    shell: /bin/bash
    hashed_passwd: $6$ViQQI4kReDWrps3y$TSzStkHkxYKn6VlaWRVrhwZSsoVe3nxolR58Iz0063Rp1Ba4w8gFtep9uy4N8cpmcF7Ey0fYrm2lmcYEHT.E./
    lock_passwd: false
    sudo: ALL=(ALL) NOPASSWD:ALL

the openstack keypair was not added to ubuntu user and rather was added only to the root user.

This was fixed once I noticed I needed to put -default on the top of the users entry ... i.e.

users:
  - default
  - name: ubuntu    
    shell: /bin/bash
    hashed_passwd: $6$ViQQI4kReDWrps3y$TSzStkHkxYKn6VlaWRVrhwZSsoVe3nxolR58Iz0063Rp1Ba4w8gFtep9uy4N8cpmcF7Ey0fYrm2lmcYEHT.E./
    lock_passwd: false
    sudo: ALL=(ALL) NOPASSWD:ALL

The cloud-init examples have the -default entry but do not explicitly mention the need to put it. Also,

Image used: https://cloud-images.ubuntu.com/jammy/20251216/jammy-server-cloudimg-amd64.img

ubuntu@vmwithpassword:~$ cloud-init --version
/usr/bin/cloud-init 25.2-0ubuntu1~22.04.1

ubuntu@vmwithpassword:~$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 22.04.5 LTS
Release:	22.04
Codename:	jammy

[mostafaCamel]

Not sure if this is pedantic so feel free to close if this is not needed

[mostafaCamel]

fwiw, sudo virt-cat -a ubuntu-22.04-server-cloudimg-amd64.img /etc/passwd | grep ubuntu and sudo virt-cat -a ubuntu-22.04-server-cloudimg-amd64.img /etc/shadow | grep ubuntu return nothing, which mean that the image by iitself does not have the user and that the user is created later (early during cloud-init)

[blackboxsw]

Thank you for filing this bug @mostafaCamel there is currently a reference in docs on the Users and Groups module reference at https://cloudinit.readthedocs.io/en/latest/reference/modules.html#users-and-groups which states:

Omission of ``default`` as the first item in the ``users`` list skips creation the default user

I could see that content not being conspicuous enough. If we want to convert that text to a note, or potentially also add a comment line in the all examples Including users and groups section line 15 or line 143 to caution about the absence of - default I could see that being helpful