By default, the ansible-doc command will use module as the type, thus the -t or --type option followed by shell must be used to display documentation on shell.

The -l or --list option can be used to display the shells that can be used.

ansible-doc --type shell --list

cmd        Windows Command Prompt
csh        C shell (/bin/csh)
fish       fish shell (/bin/fish)
powershell Windows PowerShell
sh         POSIX shell (/bin/sh)

Let's say you want to learn about "cmd". The -s or --snippet option can be used.

ansible-doc --type inventory --snippet cmd

You can also display information about the shell module, like this. Refer to Ansible - shell module for more information on the shell module.

ansible-doc --type module --snippet shell