Fix some typos, broken links and other issues in extension docs

chrome/common/extensions/docs/templates/articles/xhr.html

 <h1>Cross-Origin XMLHttpRequest</h1>
 
 
 <p id="classSummary">
 Regular web pages can use the
 <a href="http://www.w3.org/TR/XMLHttpRequest/">XMLHttpRequest</a>
 object to send and receive data from remote servers,
 but they're limited by the
 <a href="http://en.wikipedia.org/wiki/Same_origin_policy">same origin policy</a>.
 Extensions aren't so limited.
@@ skipping 17 matching lines @@
 
 <p>If the extension attempts to use a security origin other than itself,
 say http://www.google.com,
 the browser disallows it
 unless the extension has requested the appropriate cross-origin permissions.
 </p>
 
 <h2 id="requesting-permission">Requesting cross-origin permissions</h2>
 
 <p>By adding hosts or host match patterns (or both) to the
-<a href="manifest.html#permissions">permissions</a> section of the
+<a href="declare_permissions.html">permissions</a> section of the
 <a href="manifest.html">manifest</a> file, the extension can request access to
 remote servers outside of its origin.</p>
 
 <pre>{
   "name": "My extension",
   ...
   <b>"permissions": [
     "http://www.google.com/"
   ]</b>,
   ...
@@ skipping 104 matching lines @@
 
 <p>
 If you modify the default <a href="contentSecurityPolicy.html">Content
 Security Policy</a> for apps or extensions by adding a
 <code>content_security_policy</code> attribute to your manifest, you'll need to
 ensure that any hosts to which you'd like to connect are allowed. While the
 default policy doesn't restrict connections to hosts, be careful when explicitly
 adding either the <code>connect-src</code> or <code>default-src</code>
 directives.
 </p>