What is updateValueAndValidity : <p><a href="https://angular.io/docs/ts/latest/api/forms/index/FormControl-class.html" rel="noreferrer">These docs</a> state the following:</p>

<blockquote>
  <p>If emitEvent is true, this change will cause a valueChanges event on
  the FormControl to be emitted. This defaults to true (as it falls
  through to updateValueAndValidity).</p>
</blockquote>

<p>What is this <code>updateValueAndValidity</code>?</p>
