Update config file docs re new [Prefs] section

[Prefs] section used to store tab displayed when Preferences
dialogue was last closed.

Author: delphidabbler

Docs/Design/FileFormats/config.html

@@ -732,6 +732,27 @@ <h4>
   </dd>
 </dl>
 
+<h4>
+  [Prefs] section
+</h4>
+
+<p>
+  Stores information about the Preferences dialogue box itself, rather than actual preferences data. Actual preference data is stored in sections with names like [Prefs:XXX] where XXX is a preferences sub-section.
+</p>
+
+<p>
+  Name / Value pairs:
+</p>
+
+<dl class="indent">
+  <dt>
+    <code class="key">LastTab</code> (string)
+  </dt>
+  <dd>
+    Name of the tab that was open when the dialogue box was last closed. May be the empty string if the dialogue box has not yet been opened.
+  </dd>
+</dl>
+
 <h4>
   [Prefs:CodeGen] section
 </h4>