Settings Override API: update documentation.

The documentation describes usage of the "Install Param" for external extensions.

BUG=267510

Review URL: https://codereview.chromium.org/216473005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@263125 0039d316-1c4b-4281-b951-d872f2087c98

chrome/common/extensions/api/manifest_types.json

@@ -151,7 +151,7 @@
             }
           },
           "startup_pages": {
-            "description": "URLs to be added to the list of startup pages.",
+            "description": "An array of length one containing a URL to be used as the startup page.",
             "optional": true,
             "type": "array",
             "items": {"type": "string"}

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

@@ -46,6 +46,32 @@ manifest</a>.</p>
 <code>search_provider</code>, and <code>startup_pages</code> are only enabled
 in the Chrome Dev release.</p>
 
+<h2 id="customizing">Customizing values</h2>
+<p>Values in the manifest can be customized in the following ways:</p>
+<ul>
+  <li>
+  All values of the <code>search_provider</code>, <code>homepage</code> and
+  <code>startup_pages</code> properties can be localized using the
+  $(ref:i18n chrome.i18n API).
+  </li>
+  <li>
+  For <a href="http://developer.chrome.com/extensions/external_extensions">external extensions</a>,
+  the <code>search_provider</code>, <code>homepage</code> and
+  <code>startup_pages</code> URL values can be parametrized using a registry key.
+  A new registry entry should be created next to the "update_url" key (see
+  instructions <a href="http://developer.chrome.com/extensions/external_extensions#registry">here</a>).
+  The value name is "install_parameter", the value data is an arbitrary string:
+  <pre>{
+    "update_url": "https://clients2.google.com/service/update2/crx",
+    "install_parameter": "Value"
+  }</pre>
+  All occurrences of the substring "__PARAM__" in the manifest URLs will be
+  substituted with the "install_parameter" value. If “install_parameter” is
+  absent, occurrences of "__PARAM__" are removed. Note that "__PARAM__" cannot
+  be part of the hostname. It needs to occur after the first '/' in the URL.
+  </li>
+</ul>
+
 <h2 id="reference">Reference</h2>
 <p>
 An extension can override one or more of the following properties in the