By default, the ansible-doc command will use module as the type, thus the -t or --type option followed by connection must be used to display documentation on the different type of connections that can be used.

The -l or --list option can be used to display the connections that can be used. Refer to Ansible - Getting Started with SSH.

ansible-doc --type connection

buildah      Interact with an existing buildah container
chroot       Interact with local chroot
docker       Run tasks in docker containers
funcd        Use funcd to connect to target
httpapi      Use httpapi to run command on network appliances
iocage       Run tasks in iocage jails
jail         Run tasks in jails
kubectl      Execute tasks in pods running on Kubernetes
libvirt_lxc  Run tasks in lxc containers via libvirt
local        execute on controller
lxc          Run tasks in lxc containers via lxc python library
lxd          Run tasks in lxc containers via lxc CLI
napalm       Provides persistent connection using NAPALM
netconf      Provides a persistent connection using the netconf protocol
network_cli  Use network_cli to run command on network appliances
oc           Execute tasks in pods running on OpenShift
paramiko_ssh Run tasks via python ssh (paramiko)
persistent   Use a persistent unix socket for connection
podman       Interact with an existing podman container
psrp         Run tasks over Microsoft PowerShell Remoting Protocol
qubes        Interact with an existing QubesOS AppVM
saltstack    Allow ansible to piggyback on salt minions
ssh          connect via ssh client binary
vmware_tools Execute tasks inside a VM via VMware Tools
winrm        Run tasks over Microsoft's WinRM
zone         Run tasks in a zone instance

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

ansible-doc --type connection --snippet local

> LOCAL    (/usr/local/lib/python3.6/site-packages/ansible/plugins/connection/local.py)

        This connection plugin allows ansible to execute tasks on the Ansible 'controller' instead of on a remote host.

  * This module is maintained by The Ansible Community
NOTES:
      * The remote user is ignored, the user with which the ansible CLI was executed is used instead.


AUTHOR: ansible (@core)
        METADATA:
          status:
          - preview
          supported_by: community