### 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
"""
    This is the Alpine User manipulation class. It inherits the BusyBox class
    behaviors such as using adduser and deluser commands.
    """
    platform = 'Linux'
    distribution = 'Alpine'
def main():
    ssh_defaults = dict(
        bits=0,
        type='rsa',
        passphrase=None,
        comment='ansible-generated on %s' % socket.gethostname()
    )
    module = AnsibleModule(
        argument_spec=dict(
            state=dict(type='str', default='present', choices=['absent', 'present']),
            name=dict(type='str', required=True, aliases=['user']),
            uid=dict(type='int'),