##### 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 -->

cmd = [self.cmd_path, '--version']
        (rc, out, err) = self.module.run_command(cmd)
        tar_type = None
        if out.startswith('bsdtar'):
            tar_type = 'bsd'
        elif out.startswith('tar') and 'GNU' in out:
            tar_type = 'gnu'
        return tar_type
    @property
    def files_in_archive(self):
        if self._files_in_archive:
            return self._files_in_archive
        cmd = [self.cmd_path, '--list', '-C', self.b_dest]
        if self.zipflag:
            cmd.append(self.zipflag)
        if self.opts:
            cmd.extend(['--show-transformed-names'] + self.opts)
        if self.excludes:
            cmd.extend(['--exclude=' + f for f in self.excludes])
        cmd.extend(['-f', self.src])
        rc, out, err = self.module.run_command(cmd, cwd=self.b_dest, environ_update=dict(LANG='C', LC_ALL='C', LC_MESSAGES='C'))
        if rc != 0:
            raise UnarchiveError('Unable to list files in the archive')
        for filename in out.splitlines():
            
            
            filename = to_native(codecs.escape_decode(filename)[0])