CRM-12880 and CRM-11784 help text improvements.

----------------------------------------
* CRM-12880: ACL help text could be slightly clearer?
  http://issues.civicrm.org/jira/browse/CRM-12880
* CRM-11784: user text for civicrm/event/manage
  http://issues.civicrm.org/jira/browse/CRM-11784