docs(ngDisabled): clarify "don't do this" example

It's not clear until you read the whole thing that it's an explanation
of what *not* to do and why, so if you scan the page from the top, you
may use this bad solution.

Author: henrik

src/ng/directive/booleanAttrs.js

@@ -160,7 +160,7 @@
  *
  * @description
  *
- * The following markup will make the button enabled on Chrome/Firefox but not on IE8 and older IEs:
+ * We shouldn't do this, because it will make the button enabled on Chrome/Firefox but not on IE8 and older IEs:
  * ```html
  * <div ng-init="scope = { isDisabled: false }">
  *  <button disabled="{{scope.isDisabled}}">Disabled</button>