<!--- 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"
```

continue
            fragment = u"%s/%s" % (src_path, f)
            if not os.path.isfile(fragment) or (ignore_hidden and os.path.basename(fragment).startswith('.')):
                continue
            with open(self._loader.get_real_file(fragment, decrypt=decrypt), 'rb') as fragment_fh:
                fragment_content = fragment_fh.read()
            
            if add_newline:
                tmp.write(b'\n')
            
            if delimit_me:
                if delimiter:
                    
                    delimiter = codecs.escape_decode(delimiter)[0]
                    tmp.write(delimiter)
                    
                    
                    if delimiter[-1] != b'\n':
                        tmp.write(b'\n')
            tmp.write(fragment_content)
            delimit_me = True
            if fragment_content.endswith(b'\n'):
                add_newline = False
            else:
                add_newline = True
        tmp.close()
        return temp_path
    def run(self, tmp=None, task_vars=None):
        self._supports_check_mode = False
        result = super(ActionModule, self).run(tmp, task_vars)