addDocLink_(parentDiv, docLink) {
    const link = /** @type {!HTMLAnchorElement} */(document.createElement('a'));
    link.href = docLink;
    link.target = '_blank';
    link.classList.add('mdl-button');
    link.classList.add('mdl-js-button');
    link.classList.add('mdl-js-ripple-effect');
    link.classList.add('mdl-button--colored');

    const icon = document.createElement('i');
    icon.classList.add('material-icons-round');
    icon.textContent = 'help';

    link.appendChild(icon);
    parentDiv.appendChild(link);
  }
Creates a link that links to a section within the Shaka Player docs.
@param {!Element} parentDiv
@param {string} docLink
@private