##### SUMMARY
The unarchive module is missing documentation on its return values.
Only the `list_files` option mentions it at all:
> If set to True, return the list of files that are contained in the tarball.

It does not mention how this list is returned (Spoiler: It's in the `files` key, as a list of file names).
A `RETURN` section should be added to the module documentation.
Ideally, the `EXAMPLES` section should be extended to include one or two examples that use the return values somehow.

##### ISSUE TYPE
- Documentation Report

##### COMPONENT NAME
modules/files/unarchive.py

##### ANSIBLE VERSION

```
ansible 2.9.2
  config file = None
  configured module search path = ['/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib64/python3.6/site-packages/ansible
  executable location = /usr/lib/python-exec/python3.6/ansible
  python version = 3.6.9 (default, Oct  5 2019, 11:39:46) [GCC 8.3.0]
```

##### CONFIGURATION
not relevant

##### OS / ENVIRONMENT
not relevant

##### ADDITIONAL INFORMATION
<!--- Describe how this improves the documentation, e.g. before/after situation or screenshots -->

<!--- HINT: You can paste gist.github.com links for larger files -->

elif uid and uid != fut_uid:
                change = True
                err += 'Path %s is owned by uid %s, not by uid %s as expected\n' % (path, uid, fut_uid)
                itemized[6] = 'o'
            
            group = gid = None
            try:
                group = grp.getgrgid(st.st_gid).gr_name
            except (KeyError, ValueError, OverflowError):
                gid = st.st_gid
            if run_uid != 0 and (fut_group != run_group or fut_gid != run_gid) and fut_gid not in groups:
                raise UnarchiveError('Cannot change group ownership of %s to %s, as user %s' % (path, fut_group, run_owner))
            if group and group != fut_group:
                change = True
                err += 'Path %s is owned by group %s, not by group %s as expected\n' % (path, group, fut_group)
                itemized[6] = 'g'
            elif gid and gid != fut_gid:
                change = True
                err += 'Path %s is owned by gid %s, not by gid %s as expected\n' % (path, gid, fut_gid)
                itemized[6] = 'g'
            
            if change:
                if path not in self.includes:
                    self.includes.append(path)
                diff += '%s %s\n' % (''.join(itemized), path)
        if self.includes:
            unarchived = False
        
        return dict(unarchived=unarchived, rc=rc, out=out, err=err, cmd=cmd, diff=diff)
    def unarchive(self):