### 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.groups is None:
            return None
        info = self.user_info()
        groups = set(x.strip() for x in self.groups.split(',') if x)
        for g in groups.copy():
            if not self.group_exists(g):
                self.module.fail_json(msg="Group %s does not exist" % (g))
            if info and remove_existing and self.group_info(g)[2] == info[3]:
                groups.remove(g)
        return groups
    def user_group_membership(self, exclude_primary=True):
        ''' Return a list of groups the user belongs to '''
        groups = []
        info = self.get_pwd_info()
        for group in grp.getgrall():
            if self.name in group.gr_mem:
                
                if not exclude_primary:
                    groups.append(group[0])
                else:
                    if info[3] != group.gr_gid:
                        groups.append(group[0])
        return groups
    def user_exists(self):