Clarify krb5_rd_req documentation

For the user-to-user case, document that callers should pass a server
principal to krb5_rd_req.  For the keytab case, more accurately
document which keytab keys are tried against the ticket.

ticket: 7641 (new)
target_version: 1.11.3
tags: pullup
Fix krb5_rd_req usage in user-to-user sample app

Pass the server principal to krb5_rd_req so it can do proper transited
checking.