docs(tutorial/step_6): fix experiment description

In the experiments section, it is suggested to add a `-` symbol to
`<option value="age">Oldest</option>`. This change is made to reverse
the sort order when selecting the `age` option.

However, this change affects the default `$ctrl.orderProp` that we had
set in `phone-list.component.js`. After making the change, our default
when refreshing the page is "Sort by: [blank]".

This commit adds some additional documentation to clarify that this
behavior makes sense and that the reader should try and fix this within
`phone-list.component.js`.

Closes #16781

docs/content/tutorial/step_06.ngdoc

@@ -230,6 +230,8 @@ You can now rerun `npm run protractor` to see the tests run.
 
 * Reverse the sort order by adding a `-` symbol before the sorting value:
   `<option value="-age">Oldest</option>`
+  After making this change, you'll notice that the drop-down list has a blank option selected and does not default to age anymore.
+  Fix this by updating the `orderProp` value in `phone-list.component.js` to match the new value on the `<option>` element.
 
 
 ## Summary