### 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
if self.shell is not None and info[6] != self.shell:
            cmd.append('-s')
            cmd.append(self.shell)
        if self.login_class is not None:
            
            user_login_class = None
            userinfo_cmd = [self.module.get_bin_path('userinfo', True), self.name]
            (rc, out, err) = self.execute_command(userinfo_cmd, obey_checkmode=False)
            for line in out.splitlines():
                tokens = line.split()
                if tokens[0] == 'class' and len(tokens) == 2:
                    user_login_class = tokens[1]
            
            if self.login_class != user_login_class:
                cmd.append('-L')
                cmd.append(self.login_class)
        if self.password_lock and not info[1].startswith('*'):
            cmd.append('-Z')
        elif self.password_lock is False and info[1].startswith('*'):
            cmd.append('-U')
        if self.update_password == 'always' and self.password is not None \
                and self.password != '*' and info[1] != self.password:
            cmd.append('-p')
            cmd.append(self.password)
        
        if len(cmd) == 1:
            return (None, '', '')
        cmd.append(self.name)
        return self.execute_command(cmd)
class NetBSDUser(User):