### 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
try:
            minweeks = ''
            maxweeks = ''
            warnweeks = ''
            with open("/etc/default/passwd", 'r') as f:
                for line in f:
                    line = line.strip()
                    if (line.startswith('#') or line == ''):
                        continue
                    m = re.match(r'^([^#]*)#(.*)$', line)
                    if m:  
                        line = m.group(1)
                    key, value = line.split('=')
                    if key == "MINWEEKS":
                        minweeks = value.rstrip('\n')
                    elif key == "MAXWEEKS":
                        maxweeks = value.rstrip('\n')
                    elif key == "WARNWEEKS":
                        warnweeks = value.rstrip('\n')
        except Exception as err:
            self.module.fail_json(msg="failed to read /etc/default/passwd: %s" % to_native(err))
        return (minweeks, maxweeks, warnweeks)
    def remove_user(self):
        cmd = [self.module.get_bin_path('userdel', True)]
        if self.remove:
            cmd.append('-r')
        cmd.append(self.name)
        return self.execute_command(cmd)
    def create_user(self):