### Summary

I was surprised to after using Ansible to add some users to a group, that they had been removed from all other groups (crucially, including one used to grant them `sudo` rights!).



### Issue Type

Documentation Report

### Component Name

user_module

### Ansible Version

```console
nsible [core 2.11.5]
  config file = /redacted/git/devops-ansible/ansible.cfg
  configured module search path = ['/home/mark/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python3.9/site-packages/ansible
  ansible collection location = /home/mark/.ansible/collections:/usr/share/ansible/collections
  executable location = /sbin/ansible
  python version = 3.9.7 (default, Aug 31 2021, 13:28:12) [GCC 11.1.0]
  jinja version = 3.0.1
  libyaml = True
```


### Configuration

```console
N/A
```


### OS / Environment

Arch Linux

### Additional Information

I re-read the docs for the `user` built-in and it only mentions that the user will be *added* to groups mentioned in the groups list. The docs currently fail to mention that they will be removed from all groups *not* in the groups array.

I presume this behavior is as intended, but the docs should clarify that the user's non-primary groups will be set to exactly those in the list, any other pre-existing group memberships will be removed.

Here's a sample playbook that illustrated the behavior:

```yaml
---
- hosts: fave-server
  become: true
  become_user: root
  tasks:
    - name: "add users to some group"
      loop:
        - sue
        - john
      user:
        name: "{{ item }}"
        state: present
        groups:
          - some
```

Try having the users already exist with other group memberships and they will be removed and replace with only a `some` group membership. (I tested with Ansible 2.11.5). 

### Code of Conduct

- [X] I agree to follow the Ansible Code of Conduct
type: bool
        version_added: "2.6"
    non_unique:
        description:
            - Optionally when used with the -u option, this option allows to change the user ID to a non-unique value.
        type: bool
        default: no
        version_added: "1.1"
    seuser:
        description:
            - Optionally sets the seuser type (user_u) on selinux enabled systems.
        type: str
        version_added: "2.1"
    group:
        description:
            - Optionally sets the user's primary group (takes a group name).
        type: str
    groups:
        description:
            - List of groups user will be added to. When set to an empty string C(''),
              the user is removed from all groups except the primary group.
            - Before Ansible 2.3, the only input format allowed was a comma separated string.
        type: list
        elements: str
    append:
        description:
            - If C(yes), add the user to the groups specified in C(groups).
            - If C(no), user will only be added to the groups specified in C(groups),
              removing them from all other groups.
        type: bool