addHelpCommand(helpCommand, deprecatedDescription) {
    // If not passed an object, call through to helpCommand for backwards compatibility,
    // as addHelpCommand was originally used like helpCommand is now.
    if (typeof helpCommand !== 'object') {
      this.helpCommand(helpCommand, deprecatedDescription);
      return this;
    }

    this._addImplicitHelpCommand = true;
    this._helpCommand = helpCommand;
    this._initCommandGroup(helpCommand);
    return this;
  }
Add prepared custom help command.

@param {(Command|string|boolean)} helpCommand - custom help command, or deprecated enableOrNameAndArgs as for `.helpCommand()`
@param {string} [deprecatedDescription] - deprecated custom description used with custom name only
@return {Command} `this` command for chaining