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