Replace attachedElements() issue with note on lifetime (#65)

At TPAC the Editing WG [resolved](#59 (comment)) that `attachedElements()` should continue returning elements that are not connected.

So, delete the Issue in the spec about this and add a note that EditContext will keep its associated element alive.

Closes #59.

index.html

@@ -137,6 +137,13 @@ <h4>Association and activation</h4>
  the {{EditContext}} to the element's {{HTMLElement/editContext}} property.
  An {{HTMLElement}} can be <a data-lt="associated element">associated</a> with at most one {{EditContext}}.
  </p>
+ <p class="note">
+ An {{EditContext}} keeps its [=associated element=] alive, so developers
+ should be aware that assigning an {{EditContext}} to an element's
+ {{HTMLElement/editContext}} property will prevent the element from being garbage
+ collected until the property is cleared or the {{EditContext}} is garbage collected.
+ </p>
+ </p>
  <p>
  If an {{EditContext}}'s <a>associated element</a>'s parent is not
  <a href="https://w3c.github.io/editing/docs/execCommand/#editable">editable</a>,
@@ -1073,9 +1080,6 @@ <h3 id="editcontext-interface">EditContext Interface</h3>
  <p class="note">
  This method returns a list instead of a single element for forward compatibility if {{EditContext}} is ever granted the ability to have multiple <a>associated elements</a>.
  </p>
- <p class="issue">
- Should attachedElements() return elements that are removed from the tree? If yes, the EditContext will keep removed elements alive. If not, we probably shouldn't allow a disconnected element to <a data-lt="associated element">associate</a> with EditContext, otherwise, it would be confusing/unintuitive that we allow it but attachedElements returns empty.
- </p>
 
  <dt>ontextupdate</dt>
  <dd><p>The event handler for {{TextUpdateEvent}}.</p></dd>