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)

https://github.com/krb5/krb5/commit/84c31f692530c6be3e106e4b700c54e0c2160e30
Author: Greg Hudson <ghudson@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 84c31f692530c6be3e106e4b700c54e0c2160e30
Branch: krb5-1.11
 src/include/krb5/krb5.hin |   28 +++++++++++++++++++---------
 1 files changed, 19 insertions(+), 9 deletions(-)