docs(input): clarify that contenteditable is an HTML5 attribute

Closes angular#3841

Author: James-Dunn

src/ng/directive/input.js

@@ -856,6 +856,9 @@ var VALID_CLASS = 'ng-valid',
  * data-binding. Notice how different directives (`contenteditable`, `ng-model`, and `required`)
  * collaborate together to achieve the desired result.
  *
+ * Note that `contenteditable` is an HTML5 attribute, which tells the browser to let the element
+ * contents be edited in place by the user.  This will not work on older browsers.
+ *
  * <example module="customControl">
     <file name="style.css">
       [contenteditable] {