Add a note to tabs.onCreated documentation that the url might not be …

…defined.

Glen got bit by this during development of an extension, where he was 
expecting that the tab's url would always be defined right away.  (Someone 
filed crbug.com/52195 about this a while back, but I had marked it as WontFix 
because there are legitimate reasons why the url isn't always set)

BUG=none
TEST=none

Review URL: http://codereview.chromium.org/6193001

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@70767 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/extension_api.json

@@ -1318,7 +1318,7 @@
       {
         "name": "onCreated",
         "type": "function",
-        "description": "Fires when a tab is created.",
+        "description": "Fires when a tab is created. Note that the tab's URL may not be set at the time this event fires, but you can listen to onUpdated events to be notified when a URL is set.",
         "parameters": [
           {
             "$ref": "Tab",

chrome/common/extensions/docs/tabs.html

@@ -5303,7 +5303,7 @@ <h4>onCreated</h4>
 
               <div class="description">
                 <p class="todo" style="display: none; ">Undocumented.</p>
-                <p>Fires when a tab is created.</p>
+                <p>Fires when a tab is created. Note that the tab's URL may not be set at the time this event fires, but you can listen to onUpdated events to be notified when a URL is set.</p>
 
                 <!-- PARAMETERS -->
                 <h4>Parameters</h4>