gistfile1.txt

 $ ansible-doc -t connection ssh
 [...]
- private_key_file
        Path to private key file to use for authentication
        [Default: (null)]
        set_via:
          env:
          - name: ANSIBLE_PRIVATE_KEY_FILE
          ini:
          - key: private_key_file
            section: defaults
          vars:
          - name: ansible_private_key_file
          - name: ansible_ssh_private_key_file
        
        cli:
        - name: private_key_file

[...]