Add explicit aria-label to placeholder-only inputs

While `placeholder` is nominally valid per spec
http://rawgit.com/w3c/html-api-map/master/index.html#accessible-name-and-description-calculation
it is inelegant, a fallback, and not supported in ios/safari/voiceover
and android/chrome/talkback, to name a few combinations

`aria-describedby` is also not really the right tool for providing a
name/label equivalent, so remove that from the list of possible
alternative methods / clarify how it can be used

docs/examples/carousel/index.html

@@ -35,7 +35,7 @@
           </li>
         </ul>
         <form class="form-inline mt-2 mt-md-0">
-          <input class="form-control mr-sm-2" type="text" placeholder="Search">
+          <input class="form-control mr-sm-2" type="text" placeholder="Search" aria-label="Search">
           <button class="btn btn-outline-success my-2 my-sm-0" type="submit">Search</button>
         </form>
       </div>