Fix for issue #12

This pull request addresses the documentation clarifications outlined in issue #12.  

However, I only updated unix-system-characteristics.xsd because documentation regarding the status attribute only pertains to OVAL System Characteristics.  

Furthermore, I did not include documentation about using status="not collected" when an interpreter does not support the collection of ACL information or using status="error" when there was an error trying to retrieve ACL information because these are general cases covered in the documentation in oval-sc:StatusEnumeration.