DOC: API: add DOM query helper examples

Let's add an example usage of all of the DOM query helpers (find,
findAll, getRootElement) to the API docs.

(cherry picked from commit 7bd16c1)

API.md

@@ -493,6 +493,14 @@ Find the first element matched by the given selector. Equivalent to calling
 
 *   `selector` **[string][64]** the selector to search for
 
+#### Examples
+
+Find all of the elements matching '.my-selector'.
+
+```javascript
+findAll('.my-selector');
+```
+
 Returns **[Element][65]** matched element or null
 
 ### findAll
@@ -505,12 +513,29 @@ of a `NodeList`.
 
 *   `selector` **[string][64]** the selector to search for
 
+#### Examples
+
+Finding the first element with id 'foo'
+
+```javascript
+find('#foo');
+```
+
 Returns **[Array][70]** array of matched elements
 
 ### getRootElement
 
 Get the root element of the application under test (usually `#ember-testing`)
 
+#### Examples
+
+Getting the root element of the application and checking that it is equal
+to the element with id 'ember-testing'.
+
+```javascript
+assert.equal(getRootElement(), document.querySelector('#ember-testing'));
+```
+
 Returns **[Element][65]** the root element
 
 ## Routing Helpers
@@ -1216,23 +1241,23 @@ Returns **([Array][70]\<Warning> | [Promise][66]<[Array][70]\<Warning>>)** An ar
 
 [54]: #getdeprecations
 
-[55]: #examples-19
+[55]: #examples-22
 
 [56]: #getdeprecationsduringcallback
 
 [57]: #parameters-29
 
-[58]: #examples-20
+[58]: #examples-23
 
 [59]: #getwarnings
 
-[60]: #examples-21
+[60]: #examples-24
 
 [61]: #getwarningsduringcallback
 
 [62]: #parameters-30
 
-[63]: #examples-22
+[63]: #examples-25
 
 [64]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/String
 

addon-test-support/@ember/test-helpers/dom/find-all.ts

@@ -9,6 +9,12 @@ import { toArray } from '../ie-11-polyfills';
   @public
   @param {string} selector the selector to search for
   @return {Array} array of matched elements
+
+  @example
+  <caption>
+    Finding the first element with id 'foo'
+  </caption>
+  find('#foo');
 */
 export default function findAll(selector: string): Element[] {
   if (!selector) {

addon-test-support/@ember/test-helpers/dom/find.ts

@@ -7,6 +7,13 @@ import getElement from './-get-element';
   @public
   @param {string} selector the selector to search for
   @return {Element} matched element or null
+
+  @example
+  <caption>
+    Find all of the elements matching '.my-selector'.
+  </caption>
+  findAll('.my-selector');
+
 */
 export default function find(selector: string): Element | null {
   if (!selector) {

addon-test-support/@ember/test-helpers/dom/get-root-element.ts

@@ -6,6 +6,13 @@ import { isDocument, isElement } from './-target';
 
   @public
   @returns {Element} the root element
+
+  @example
+  <caption>
+    Getting the root element of the application and checking that it is equal
+    to the element with id 'ember-testing'.
+  </caption>
+  assert.equal(getRootElement(), document.querySelector('#ember-testing'));
 */
 export default function getRootElement(): Element | Document {
   let context = getContext();