CRM-13090 add comment on public var.

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* CRM-13090: New Petition/Survey form validation - enforce unique constraint on ActivityType-Campaign-Title
  http://issues.civicrm.org/jira/browse/CRM-13090

CRM/Campaign/Form/Petition.php

@@ -41,8 +41,9 @@
 class CRM_Campaign_Form_Petition extends CRM_Core_Form {
 
   /**
+   * Making this public so we can reference it in the formRule
    * @var int
-   * @protected
+   * @public
    */
   public $_surveyId;