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.

(cherry picked from commit 98aa233e18245981b491affe5fa70623cb83b705)

ticket: 7641
version_fixed: 1.11.3
status: resolved