Clarify immediate script abortion when user prompt appears.

index.html

@@ -6705,8 +6705,8 @@ <h3>Executing Script</h3>
  The algorithm returns <a data-lt="completion">an ECMAScript completion record</a>.
 
 <p>If at any point during the algorithm a <a>user prompt</a> appears,
- abort all subsequent substeps of this algorithm, and return
- <a>Completion</a> { [[\Type]]: <code>normal</code>, [[\Value]]: <a><code>null</code></a>, [[\Target]]: <code>empty</code> }.
+ immediately return <a>Completion</a> { [[\Type]]: <code>normal</code>, [[\Value]]: <a><code>null</code></a>,
+ [[\Target]]: <code>empty</code> }, but continue to run the other steps of this algorithm in parallel.
 
 <ol>
  <li><p>Let <var>window</var> be the <a>associated window</a>