Improve help text for setfacl --set option

The help text associated with the command "setfacl --set" states:

 --set       Fully replace the ACL, discarding all existing entries. The        
              <acl_spec> must include entries for user, group, and others for    
              compatibility with permission bits.               

However the actual behaviour is a bit more subtle:

If the ACL spec contains only access entries, then the existing default entries are retained. If the ACL spec contains only default entries, then the existing access entries are retained. If the ACL spec contains both access and default entries, then both are replaced.

This Jira will improve the help text to more align with the expected behaviour.