### 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
ssh_key_passphrase=dict(type='str', no_log=True),
            update_password=dict(type='str', default='always', choices=['always', 'on_create'], no_log=False),
            expires=dict(type='float'),
            password_lock=dict(type='bool', no_log=False),
            local=dict(type='bool'),
            profile=dict(type='str'),
            authorization=dict(type='str'),
            role=dict(type='str'),
            umask=dict(type='str'),
        ),
        supports_check_mode=True,
    )
    user = User(module)
    user.check_password_encrypted()
    module.debug('User instantiated - platform %s' % user.platform)
    if user.distribution:
        module.debug('User instantiated - distribution %s' % user.distribution)
    rc = None
    out = ''
    err = ''
    result = {}
    result['name'] = user.name
    result['state'] = user.state
    if user.state == 'absent':
        if user.user_exists():
            if module.check_mode:
                module.exit_json(changed=True)
            (rc, out, err) = user.remove_user()
            if rc != 0:
                module.fail_json(name=user.name, msg=err, rc=rc)