Function cmd_man 

pub fn cmd_man(
    command_name: &str,
    root_cmd: &Command,
    prog_name: &str,
    version: &str,
) -> Result<String, ShellError>

Handler: look up a subcommand and return its roff man page.

Returns Err(ShellError::UnknownCommand) if command_name is not found among root_cmd’s subcommands and is not in KNOWN_BUILTINS.