##### 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 -->

if filename.startswith('/'):
                filename = filename[1:]
            exclude_flag = False
            if self.excludes:
                for exclude in self.excludes:
                    if fnmatch.fnmatch(filename, exclude):
                        exclude_flag = True
                        break
            if not exclude_flag:
                self._files_in_archive.append(to_native(filename))
        return self._files_in_archive
    def is_unarchived(self):
        cmd = [self.cmd_path, '--diff', '-C', self.b_dest]
        if self.zipflag:
            cmd.append(self.zipflag)
        if self.opts:
            cmd.extend(['--show-transformed-names'] + self.opts)
        if self.file_args['owner']:
            cmd.append('--owner=' + quote(self.file_args['owner']))
        if self.file_args['group']:
            cmd.append('--group=' + quote(self.file_args['group']))
        if self.module.params['keep_newer']:
            cmd.append('--keep-newer-files')
        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'))