fix gaps in user manual section on Kerberos for clients

clean up several gaps in the docs that I stumbled on while setting up a cluster with kerberos for clients:

• document setting up a superuser on an existing system that's enabling kerberos
• include example of verifying access for the superuser
• per-user client configuration gives wrong filename
• per-user client configuration required additions for kerberos missing rpc.sasl.qop
• document trace.token.property.keytab
• note required permissions for trace user (Table.READ, Table.WRITE, Table.ALTER_TABLE)