Document using tokens in tracked URL

If/when https://github.com/civicrm/org.civicrm.flexmailer/pull/46 is merged and released, we need to parse out a summary from https://github.com/civicrm/org.civicrm.flexmailer/pull/46 and check https://github.com/civicrm/org.civicrm.flexmailer/issues/30 to be able to update the documention on this new feature.