Clarify broker_krb_service_name documentation

broker_krb_service_name is prefxied with 'HTTP/' string and used by mod_auth_krb
where as bind_krb_principal is expected to be the full principal name. We could
normalize those two but for now just document the proper use.
Merge pull request #364 from sdodson/docs-broker_krb_service_name

Clarify broker_krb_service_name documentation