Dynamically generate a heading for Extension Docs API pages

chrome/common/extensions/api/declarative_content.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "declarativeContent",
+    "description": "Use the <code>chrome.declarativeContent</code> module to take actions depending on the content of a page, without requiring permission to read the page's content.",
     "documentation_permissions_required": ["declarative", "declarativeContent"],
     "types": [
       {
         "id": "PageStateMatcher",
         "type": "object",
         "description": "Matches the state of a web page by various criteria.",
         "properties": {
           "pageUrl": {
             "$ref": "events.UrlFilter",
             "description": "Matches if the condition of the UrlFilter are fulfilled for the top-level URL of the page.",
@@ skipping 46 matching lines @@
           "supportsRules": true,
           "conditions": ["declarativeContent.PageStateMatcher"],
           "actions": [
             "declarativeContent.ShowPageAction"
           ]
         }
       }
     ]
   }
 ]