title: Proper way to delete-if-exists?
body: <p>I have a use case where I need to delete a document that may or may not exist. I delete like this</p>
  <pre><code>db.collection('cities').doc('DC').delete();
  </code></pre>
  <p>It works fine, but when the document doesn't exist, I get an error in the log, which I'd rather not see. I could read the document first to ensure it exists, but that seems wasteful. What's the proper delete-if-exists approach?</p>
  <p>EDIT I'm performing this operation using Cloud Functions (JS)</p>