##### 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 -->

def __init__(self, src, b_dest, file_args, module):
        self.src = src
        self.b_dest = b_dest
        self.file_args = file_args
        self.opts = module.params['extra_opts']
        self.module = module
        if self.module.check_mode:
            self.module.exit_json(skipped=True, msg="remote module (%s) does not support check mode when using gtar" % self.module._name)
        self.excludes = [path.rstrip('/') for path in self.module.params['exclude']]
        
        self.cmd_path = self.module.get_bin_path('gtar', None)
        if not self.cmd_path:
            
            self.cmd_path = self.module.get_bin_path('tar')
        self.zipflag = '-z'
        self._files_in_archive = []
        if self.cmd_path:
            self.tar_type = self._get_tar_type()
        else:
            self.tar_type = None
    def _get_tar_type(self):