docs: clarify default user entry in users examples

[alan747271363-art]

Summary

• turn the existing default-user behavior warning into a more visible note in the Users and Groups module docs
• add the same warning to the YAML example page so custom users: lists mention keeping default first

Fixes #6702.

Testing

• git diff --check HEAD~1..HEAD
• YAML parse check for doc/module-docs/cc_users_groups/data.yaml

Note: I could not run the full Sphinx docs build on Windows because importing cloud-init docs currently requires the Unix-only grp module in this environment.

[holmanb]

Isn't this already documented?

[blackboxsw]

This was documented already, but with less visibility as inline documentation. This doc set provides more detail around the impact of omitting the - default user (like SSH keys being absent) which I don't think we document currently.

I'm +1 on creating this separate note to aid in visibility of the side-effects of omitting default.

[Copilot]

Pull request overview

This PR updates cloud-init documentation to more explicitly warn that when providing a custom users: list, - default must remain the first entry to ensure the distro default user is created and receives cloud-provided SSH keys (addressing issue #6702).

Changes:

• Adds a prominent note in the cc_users_groups module documentation describing the - default ordering requirement.
• Adds the same warning to the YAML examples page for users/groups.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
doc/rtd/reference/yaml_examples/user_groups.rst	Adds an explicit warning in the “Add users to the system” example section about keeping default first in users:.
doc/module-docs/cc_users_groups/data.yaml	Converts the prior inline mention into a more visible .. note:: about keeping default first in users: lists.

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[blackboxsw]

@alan747271363-art thank you for this submission. I believe this update improved visibility to the side-effects of removing - default setting from the users configuration in cloud-config.