Fix and document onUpdate

Author: marijnh

lib/codemirror.js

@@ -1718,6 +1718,7 @@ var CodeMirror = (function() {
     onCursorActivity: null,
     onGutterClick: null,
     onHighlightComplete: null,
+    onUpdate: null,
     onFocus: null, onBlur: null, onScroll: null,
     matchBrackets: false,
     workTime: 100,

manual.html

@@ -234,6 +234,9 @@ <h2 id="config">Configuration</h2>
       this function (if given) will be called. It'll be given a single
       argument, the editor instance.</dd>
 
+      <dt id="option_onUpdate"><code>onUpdate (function)</code></dt>
+      <dd>Will be called whenever CodeMirror updates its DOM display.</dd>
+
       <dt id="option_matchBrackets"><code>matchBrackets (boolean)</code></dt>
       <dd>Determines whether brackets are matched whenever the cursor
       is moved next to a bracket.</dd>