##### 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 self._infodict:
            return self._infodict[path]
        try:
            archive = ZipFile(self.src)
        except BadZipfile as e:
            if e.args[0].lower().startswith('bad magic number'):
                
                
                self._legacy_file_list()
            else:
                raise
        else:
            try:
                for item in archive.infolist():
                    self._infodict[item.filename] = int(item.CRC)
            except Exception:
                archive.close()
                raise UnarchiveError('Unable to list files in the archive')
        return self._infodict[path]
    @property
    def files_in_archive(self):