##### 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 -->

''' Convert a Unix permission string (rw-r--r--) into a mode (0644) '''
        revstr = modestr[::-1]
        mode = 0
        for j in range(0, 3):
            for i in range(0, 3):
                if revstr[i + 3 * j] in ['r', 'w', 'x', 's', 't']:
                    mode += 2 ** (i + 3 * j)
        
        return (mode & ~umask)
    def _legacy_file_list(self):
        unzip_bin = self.module.get_bin_path('unzip')
        if not unzip_bin:
            raise UnarchiveError('Python Zipfile cannot read %s and unzip not found' % self.src)
        rc, out, err = self.module.run_command([unzip_bin, '-v', self.src])
        if rc:
            raise UnarchiveError('Neither python zipfile nor unzip can read %s' % self.src)
        for line in out.splitlines()[3:-2]:
            fields = line.split(None, 7)
            self._files_in_archive.append(fields[7])
            self._infodict[fields[7]] = int(fields[6])
    def _crc32(self, path):