Update docs mentioning a unique id for an empty state in a stream

[Aratramba]

Currently the docs do not mention that you need an id when adding an empty state inside the stream DOM element. It is added in the code example.

This PR updates the wording above the example.

Reasoning

When the stream is updated the empty state will be added multiple times if it does not have an id. Therefore the :only selector will not work. Makes perfect sense, but it would be good to call it out in the docs.

Example

if you forget to add the ID here it will not work

<tbody id="songs" phx-update="stream">
-  <tr class="only:block hidden">
 + <tr id="songs-empty" class="only:block hidden">
      <td colspan="2">No songs found</td>
  </tr>
</tbody>