Add "Editor's Note" about missing element reflection attributes #1318

The IDL Interface section needs an "Editor's Note" about the missing element reflection attributes in the mixin and/or the correspondence table. (To see the Correspondence table, please wait until after #984 is merged).

Here's an example Editor's Note in combobox and the corresponding markup on index.html#L2098:

<p class="ednote" title="Major Changes to combobox role in ARIA 1.2">
The Guidance for <rref>combobox</rref> has changed significantly ...
</p>