Fix FormHelper::postLink() description.

Update description with a short explanation that considers the
`inline` and `block` options.

lib/Cake/View/Helper/FormHelper.php

@@ -1793,8 +1793,12 @@ public function postButton($title, $url, $options = array()) {
  * Creates an HTML link, but access the URL using the method you specify (defaults to POST).
  * Requires javascript to be enabled in browser.
  *
- * This method creates a `<form>` element. So do not use this method inside an existing form.
- * Instead you should add a submit button using FormHelper::submit()
+ * This method creates a `<form>` element. If you want to use this method inside of an
+ * existing form, you must use the `inline` or `block` options so that the new form is
+ * being set to a view block that can be rendered outside of the main form.
+ *
+ * If all you are looking for is a button to submit your form, then you should use
+ * `FormHelper::submit()` instead.
  *
  * ### Options:
  *