### 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
cmd.append(self.ssh_bits)
        cmd.append('-C')
        cmd.append(self.ssh_comment)
        cmd.append('-f')
        cmd.append(ssh_key_file)
        if self.ssh_passphrase is not None:
            if self.module.check_mode:
                self.module.debug('In check mode, would have run: "%s"' % cmd)
                return (0, '', '')
            master_in_fd, slave_in_fd = pty.openpty()
            master_out_fd, slave_out_fd = pty.openpty()
            master_err_fd, slave_err_fd = pty.openpty()
            env = os.environ.copy()
            env['LC_ALL'] = get_best_parsable_locale(self.module)
            try:
                p = subprocess.Popen([to_bytes(c) for c in cmd],
                                     stdin=slave_in_fd,
                                     stdout=slave_out_fd,
                                     stderr=slave_err_fd,
                                     preexec_fn=os.setsid,
                                     env=env)
                out_buffer = b''
                err_buffer = b''
                while p.poll() is None:
                    r, w, e = select.select([master_out_fd, master_err_fd], [], [], 1)
                    first_prompt = b'Enter passphrase (empty for no passphrase):'
                    second_prompt = b'Enter same passphrase again'
                    prompt = first_prompt
                    for fd in r:
                        if fd == master_out_fd: