<!--- Verify first that your issue is not already reported on GitHub -->
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##### SUMMARY
<!--- Explain the problem briefly below -->
I would like to assemble files into one with `assemble` module. Documentation said it can decrypt files. After a few tests with `Ansible 2.9` I discover that parameter not implemented.

That argument should be available since `Ansible 2.4`. Which is wrong, documentation or the module?

##### ISSUE TYPE
- Bug Report
- Documentation

##### COMPONENT NAME
<!--- Write the short name of the module, plugin, task or feature below, use your best guess if unsure -->
`assemble`, core module.

##### ANSIBLE VERSION
<!--- Paste verbatim output from "ansible --version" between quotes -->
```paste below
ansible 2.9.0
  config file = None
  configured module search path = ['/home/antonin/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python3.6/site-packages/ansible
  executable location = /bin/ansible
  python version = 3.6.9 (default, Nov 26 2019, 16:36:51) [GCC 9.2.0]
```

##### CONFIGURATION
<!--- Paste verbatim output from "ansible-config dump --only-changed" between quotes -->
```paste below
n/a — Empty
```

##### OS / ENVIRONMENT
<!--- Provide all relevant information below, e.g. target OS versions, network device firmware, etc. -->
Local task on Voidlinux with `linux 5.3.14_1`.

##### STEPS TO REPRODUCE
<!--- Describe exactly how to reproduce the problem, using a minimal test-case -->

<!--- Paste example playbooks or commands between quotes below -->

```yaml
- name: Tinker certificate
  delegate_to: localhost
  assemble:
    decrypt: true
    src: 'certificates/'
    dest: 'generated/foo.pem'
```
With encrypted files in `certificates/` directory.

<!--- HINT: You can paste gist.github.com links for larger files -->

##### EXPECTED RESULTS
<!--- Describe what you expected to happen when running the steps above -->
A file should be created from other files assemble, like `cat file1 file2 file3 > new_file` but uncrypted.

##### ACTUAL RESULTS
<!--- Describe what actually happened. If possible run with extra verbosity (-vvvv) -->

<!--- Paste verbatim command output between quotes -->
```paste below
"msg": "Unsupported parameters for (assemble) module: decrypt Supported parameters include: attributes, backup, content, delimiter, dest, directory_mode, follow, force, group, ignore_hidden, mode, owner, regexp, remote_src, selevel, serole, setype, seuser, src, unsafe_writes, validate"
```

dest = self._remote_expand_user(dest)
            dest_stat = self._execute_remote_stat(dest, all_vars=task_vars, follow=follow)
            diff = {}
            
            new_module_args = self._task.args.copy()
            
            for opt in ['remote_src', 'regexp', 'delimiter', 'ignore_hidden', 'decrypt']:
                if opt in new_module_args:
                    del new_module_args[opt]
            new_module_args['dest'] = dest
            if path_checksum != dest_stat['checksum']:
                if self._play_context.diff:
                    diff = self._get_diff_data(dest, path, task_vars)
                remote_path = self._connection._shell.join_path(self._connection._shell.tmpdir, 'src')
                xfered = self._transfer_file(path, remote_path)
                
                self._fixup_perms2((self._connection._shell.tmpdir, remote_path))
                new_module_args.update(dict(src=xfered,))
                res = self._execute_module(module_name='copy', module_args=new_module_args, task_vars=task_vars)
                if diff:
                    res['diff'] = diff
                result.update(res)
            else:
                result.update(self._execute_module(module_name='file', module_args=new_module_args, task_vars=task_vars))
        except AnsibleAction as e:
            result.update(e.result)
        finally:
            self._remove_tmp_path(self._connection._shell.tmpdir)
        return result