Extension Docs: Document asyncBlocking in the Web Request API Docs.

chrome/common/extensions/docs/templates/intros/webRequest.html

 <h2 id="manifest">Manifest</h2>
 <p>You must declare the "webRequest" permission in the <a
   href="manifest">extension manifest</a> to use the web request
 API, along with <a href="declare_permissions">host permissions</a>
 for any hosts whose network requests you want to access. If you want to
 use the web request API in a blocking fashion, you need to request
 the "webRequestBlocking" permission in addition.
 For example:</p>
 <pre data-filename="manifest.json">
 {
@@ skipping 192 matching lines @@
 <code>'blocking'</code> (only allowed for specific events), the callback
 function is handled synchronously. That means that the request is blocked until
 the callback function returns. In this case, the callback can return a
   $(ref:webRequest.BlockingResponse) that determines the further
 life cycle of the request. Depending on the context, this response allows
 cancelling or redirecting a request (<code>onBeforeRequest</code>), cancelling a
 request or modifying headers (<code>onBeforeSendHeaders</code>,
 <code>onHeadersReceived</code>), or providing authentication credentials
 (<code>onAuthRequired</code>).</p>
 
+<p>If the optional <code>opt_extraInfoSpec</code> array contains the string
+<code>'asyncBlocking'</code> instead (only allowed for
+<code>onAuthRequired</code>), the extension can generate the
+$(ref:webRequest.BlockingResponse) asynchronously.
+
 <p>The $(ref:webRequest.RequestFilter)
 <code>filter</code> allows limiting the requests for which events are
 triggered in various dimensions:
 <dl>
   <dt>URLs</dt>
   <dd><a href="match_patterns">URL patterns</a> such as
   <code>*://www.google.com/foo*bar</code>.</dd>
   <dt>Types</dt>
   <dd>Request types such as <code>main_frame</code> (a document that is loaded
   for a top-level frame), <code>sub_frame</code> (a document that is loaded for
@@ skipping 96 matching lines @@
       }
     }
     return {requestHeaders: details.requestHeaders};
   },
   {urls: ["&lt;all_urls&gt;"]},
   ["blocking", "requestHeaders"]);
 </pre>
 
 <p> For more example code, see the <a href="samples#search:webrequest">web request
 samples</a>.</p>