Add clarity around app_guid
Add clarity around app_guid

Just explain why it might not be present in the message

Signed-off-by: Doug Davis <ff3bde741799c0eff815b83a54eda3fc494df54f@us.ibm.com>

servicebroker/servicebroker,duglin/servicebroker,servicebroker/servicebroker,openservicebrokerapi/servicebroker,servicebroker/servicebroker