ipa-getkeytab man page: add more details about the -r option
The man page does not provide enough information about replicated
environments and the use of the -r option.
This fix adds an example how to use the same keytab on 2 different
hosts, and points to ipa {service/host}-allow-retrieve-keytab.
Fixes:
https://pagure.io/freeipa/issue/7237
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>