docs: cc_users_groups: add more explicit docs on impact of omission of "- default" user on a system

This bug was originally filed in Launchpad as [LP: #1943275](https://bugs.launchpad.net/cloud-init/+bug/1943275)

Launchpad details

affected_projects = []

assignee = None

assignee_name = None

date_closed = None

date_created = 2021-09-10T17:34:53.078840+00:00

date_fix_committed = None

date_fix_released = None

id = 1943275

importance = low

is_complete = False

lp_url = https://bugs.launchpad.net/cloud-init/+bug/1943275

milestone = None

owner = chad.smith

owner_name = Chad Smith

private = False

status = triaged

submitter = chad.smith

submitter_name = Chad Smith

tags = ['docs']

duplicates = []

_Launchpad user **Chad Smith(chad.smith)** wrote on 2021-09-10T17:34:53.078840+00:00_

https://cloudinit.readthedocs.io/en/latest/topics/modules.html#users-and-groups should have a more visible note explaing the affect of omitting "- default" as this is a fairly frequent accidental side-effect for user-config containing "users:" directives.

Start with the cloud-init documentation section linked in the issue, “Users and Groups,” and review how the existing users: directive documentation explains the default user. Add a visible explanation of the system impact when “- default” is omitted; the work is done when that accidental side effect is explicit to readers.

Written by the indexing model from the issue text.