### 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
return (rc, _out, _err)
        (rc, _out, _err) = self._change_user_password()
        out += _out
        err += _err
        self._update_system_user()
        
        
        if self.groups:
            (rc, _out, _err, changed) = self._modify_group()
            out += _out
            err += _err
        return (rc, out, err)
    def modify_user(self):
        changed = None
        out = ''
        err = ''
        if self.group:
            self._make_group_numerical()
        for field in self.fields:
            if field[0] in self.__dict__ and self.__dict__[field[0]]:
                current = self._get_user_property(field[1])
                if current is None or current != to_text(self.__dict__[field[0]]):
                    cmd = self._get_dscl()
                    cmd += ['-create', '/Users/%s' % self.name, field[1], self.__dict__[field[0]]]
                    (rc, _out, _err) = self.execute_command(cmd)
                    if rc != 0:
                        self.module.fail_json(
                            msg='Cannot update property "%s" for user "%s".'
                                % (field[0], self.name), err=err, out=out, rc=rc)
                    changed = rc