<!--- Verify first that your issue is not already reported on GitHub -->
<!--- Also test if the latest release and devel branch are affected too -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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"
```

del tmp  
        if task_vars is None:
            task_vars = dict()
        src = self._task.args.get('src', None)
        dest = self._task.args.get('dest', None)
        delimiter = self._task.args.get('delimiter', None)
        remote_src = self._task.args.get('remote_src', 'yes')
        regexp = self._task.args.get('regexp', None)
        follow = self._task.args.get('follow', False)
        ignore_hidden = self._task.args.get('ignore_hidden', False)
        decrypt = self._task.args.get('decrypt', True)
        try:
            if src is None or dest is None:
                raise AnsibleActionFail("src and dest are required")
            if boolean(remote_src, strict=False):
                result.update(self._execute_module(module_name='assemble', task_vars=task_vars))
                raise _AnsibleActionDone()
            else:
                try:
                    src = self._find_needle('files', src)
                except AnsibleError as e:
                    raise AnsibleActionFail(to_native(e))
            if not os.path.isdir(src):
                raise AnsibleActionFail(u"Source (%s) is not a directory" % src)
            _re = None
            if regexp is not None:
                _re = re.compile(regexp)
            
            path = self._assemble_from_fragments(src, delimiter, _re, ignore_hidden, decrypt)
            path_checksum = checksum_s(path)