Define the onabort event handler IDL attribute

Previously, the IDL was present, but there was no behavior.

dom.bs

@@ -1835,6 +1835,11 @@ service worker.
 <p>The <dfn attribute for=AbortSignal>aborted</dfn> attribute's getter, when invoked, must return
 true if the <a>context object</a>'s [=AbortSignal/aborted flag=] is set, and false otherwise.
 
+<p>The <dfn attribute for=AbortSignal><code>onabort</code></dfn> attribute is an
+<a>event handler IDL attribute</a> for the <dfn export for=AbortSignal><code>onabort</code></dfn>
+<a>event handler</a>, whose <a>event handler event type</a> is
+<dfn event for=AbortSignal><code>abort</code></dfn>.
+
 <p class=note>Changes to an {{AbortSignal}} object represent the wishes of the corresponding
 {{AbortController}} object, but an API observing the {{AbortSignal}} object can chose to ignore
 them. For instance, if the operation has already completed.
@@ -1852,7 +1857,7 @@ them. For instance, if the operation has already completed.
 
  <li><p><a for=set>Empty</a> <var>signal</var>'s <a for=AbortSignal>abort algorithms</a>.
 
- <li><p>[=Fire an event=] named <code>abort</code> at <var>signal</var>.
+ <li><p>[=Fire an event=] named {{AbortSignal/abort}} at <var>signal</var>.
 </ol>
 
 <p>A <var>followingSignal</var> (an {{AbortSignal}}) is made to