bpo-27099: Finish updating IDLE doc and help text. (#3510)

As needed for the conversion of extensions to features.

Doc/library/idle.rst

@@ -671,7 +671,7 @@ Extensions
 ^^^^^^^^^^
 
 IDLE contains an extension facility.  Preferences for extensions can be
-changed with Configure Extensions. See the beginning of config-extensions.def
-in the idlelib directory for further information.  The only current default
-extension is zoomheight. It exists as an extension primarily to be an example
-and for testing purposes.
+changed with the Extensions tab of the preferences dialog. See the
+beginning of config-extensions.def in the idlelib directory for further
+information.  The only current default extension is zzdummy, an example
+also used for testing.

Lib/idlelib/help.html

@@ -635,20 +635,11 @@ custom key set in the Configure IDLE dialog under the keys tab.</p>
 <div class="section" id="extensions">
 <h3>25.5.4.3. Extensions<a class="headerlink" href="#extensions" title="Permalink to this headline">¶</a></h3>
 <p>IDLE contains an extension facility.  Preferences for extensions can be
-changed with Configure Extensions. See the beginning of config-extensions.def
-in the idlelib directory for further information.  The default extensions
-are currently:</p>
-<ul class="simple">
-<li>FormatParagraph</li>
-<li>AutoExpand</li>
-<li>ZoomHeight</li>
-<li>ScriptBinding</li>
-<li>CallTips</li>
-<li>ParenMatch</li>
-<li>AutoComplete</li>
-<li>CodeContext</li>
-<li>RstripExtension</li>
-</ul>
+changed with the Extensions tab of the preferences dialog. See the
+beginning of config-extensions.def in the idlelib directory for further
+information.  The only current default extension is zzdummy, an example
+also used for testing.
+</p>
 </div>
 </div>
 </div>
@@ -711,7 +702,7 @@ are currently:</p>
     <ul class="this-page-menu">
       <li><a href="../bugs.html">Report a Bug</a></li>
       <li>
-        <a href="https://github.com/python/cpython/blob/master/Doc/library/idle.rst"
+        <a href="https://github.com/python/cpython/blob/master/Doc/library/idle.txt"
             rel="nofollow">Show Source
         </a>
       </li>
@@ -768,7 +759,7 @@ are currently:</p>
     The Python Software Foundation is a non-profit corporation.
     <a href="https://www.python.org/psf/donations/">Please donate.</a>
     <br />
-    Last updated on Jun 13, 2017.
+    Last updated on Sep 12, 2017.
     <a href="../bugs.html">Found a bug</a>?
     <br />
     Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.3.6.