Document markdownOptions

[Manvel]

// See https://markdown-it.github.io/markdown-it/#MarkdownIt.new
const markdownOptions = {
  highlight(str, langAndi18n)
  {
    const [lang] = langAndi18n.split("-");
    const result = (lang && getLanguage(lang)) ? highlight(lang, str).value : "";
    // Escape i18n bracket to use inside i18n strings
    return result.replace(/\\{/g, "{").replace(/\\}/g, "}");
  }
};