title: Some interfaces (e.g. IUser or IData) use "I" as the beginning of the interface name, why?
body: <p>Does this I refer to the interface or person？Such as:</p>
  <pre class="lang-js prettyprint-override"><code>interface IUser extends Document {
    email: string;
    firstName: string;
    lastName: string;
  }
  </code></pre>