<!--- 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"
```

#
#
#
#
from __future__ import (absolute_import, division, print_function)
__metaclass__ = type
import codecs
import os
import os.path
import re
import tempfile
from ansible import constants as C
from ansible.errors import AnsibleError, AnsibleAction, _AnsibleActionDone, AnsibleActionFail
from ansible.module_utils._text import to_native, to_text
from ansible.module_utils.parsing.convert_bool import boolean
from ansible.plugins.action import ActionBase
from ansible.utils.hashing import checksum_s
class ActionModule(ActionBase):
    TRANSFERS_FILES = True
    def _assemble_from_fragments(self, src_path, delimiter=None, compiled_regexp=None, ignore_hidden=False, decrypt=True):
        ''' assemble a file from a directory of fragments '''
        tmpfd, temp_path = tempfile.mkstemp(dir=C.DEFAULT_LOCAL_TMP)
        tmp = os.fdopen(tmpfd, 'wb')
        delimit_me = False
        add_newline = False
        for f in (to_text(p, errors='surrogate_or_strict') for p in sorted(os.listdir(src_path))):
            if compiled_regexp and not compiled_regexp.search(f):