Editorial: attribute tweaks for form-associated custom elements

* Adds an annotation to prevent "is" from getting over-explained, per
  whatwg/html-build#210.
* Removes the double description of the disabled attribute
  (attr-fe-disabled), once for form elements and once for
  form-associated custom elements. There is only one <dfn>
  so there should be only one attribute index entry.

source

@@ -66865,7 +66865,7 @@ document.body.append(parent);
    <dt><span data-x="concept-element-content-model">Content model</span>:</dt>
    <dd><span>Transparent</span>.</dd>
    <dt><span data-x="concept-element-attributes">Content attributes</span>:</dt>
-   <dd><span>Global attributes</span>, except the <code data-x="attr-is">is</code> attribute</dd>
+   <dd><span>Global attributes</span>, except the <code data-x="attr-is">is</code> attribute<!-- no-annotate --></dd>
    <dd><code data-x="attr-fae-form">form</code>, for <span data-x="form-associated custom element">form-associated custom elements</span></dd>
    <dd><code data-x="attr-fe-disabled">disabled</code>, for <span data-x="form-associated custom element">form-associated custom elements</span></dd>
    <dd><code data-x="attr-face-readonly">readonly</code>, for <span data-x="form-associated custom element">form-associated custom elements</span></dd>
@@ -121283,15 +121283,10 @@ interface <dfn>External</dfn> {
           <code data-x="attr-optgroup-disabled">optgroup</code>;
           <code data-x="attr-option-disabled">option</code>;
           <code data-x="attr-fe-disabled">select</code>;
-          <code data-x="attr-fe-disabled">textarea</code>
+          <code data-x="attr-fe-disabled">textarea</code>;
+          <span data-x="attr-fe-disabled">form-associated custom elements</span>
      <td> Whether the form control is disabled
      <td> <span>Boolean attribute</span>
-    <tr>
-     <th> <code data-x="">disabled</code>
-     <td> <span data-x="attr-fe-disabled">form-associated custom elements</span>
-     <td> Trigger "<code data-x="">formDisabledCallback</code>" reaction, and
-          disable form submission for the element's data.
-     <td> <span>Boolean attribute</span>
     <tr>
      <th> <code data-x="">disabled</code>
      <td> <code data-x="attr-fieldset-disabled">fieldset</code>