Add warning in documentation that the RequestContentScript action is still experimental

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> API to take actions depending on the content of a page, without requiring permission to read the page's content.",
     "types": [
       {
@@ skipping 33 matching lines @@
         "type": "object",
         "properties": {
           "instanceType": {
             "type": "string", "enum": ["declarativeContent.ShowPageAction"],
             "nodoc": true
           }
         }
       },
       {
         "id": "RequestContentScript",
-        "description": "Declarative event action that injects a content script.",
+        "description": "Declarative event action that injects a content script. <p><b>WARNING:</b> This action is still experimental and is not supported on stable builds of Chrome.</p>",
         "type": "object",
         "properties": {
           "css": {
             "type": "array",
             "optional": true,
             "description": "Names of CSS files to be injected as a part of the content script.",
             "items": { "type": "string" }
           },
           "js": {
             "type": "array",
@@ skipping 29 matching lines @@
           "conditions": ["declarativeContent.PageStateMatcher"],
           "actions": [
             "declarativeContent.RequestContentScript",
             "declarativeContent.ShowPageAction"
           ]
         }
       }
     ]
   }
 ]