### 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
maybe_invalid = True
                    else:
                        maybe_invalid = True
            if maybe_invalid:
                self.module.warn("The input password appears not to have been hashed. "
                                 "The 'password' argument must be encrypted for this module to work properly.")
    def execute_command(self, cmd, use_unsafe_shell=False, data=None, obey_checkmode=True):
        if self.module.check_mode and obey_checkmode:
            self.module.debug('In check mode, would have run: "%s"' % cmd)
            return (0, '', '')
        else:
            
            cmd = [str(x) for x in cmd]
            return self.module.run_command(cmd, use_unsafe_shell=use_unsafe_shell, data=data)
    def backup_shadow(self):
        if not self.module.check_mode and self.SHADOWFILE:
            return self.module.backup_local(self.SHADOWFILE)
    def remove_user_userdel(self):
        if self.local:
            command_name = 'luserdel'
        else:
            command_name = 'userdel'
        cmd = [self.module.get_bin_path(command_name, True)]
        if self.force and not self.local:
            cmd.append('-f')
        if self.remove:
            cmd.append('-r')
        cmd.append(self.name)
        return self.execute_command(cmd)
    def create_user_useradd(self):