doc: state klog.krb is obsolete

State upfront that klog and klog.krb (v4) are obsolete.

Update the klog.krb description and remove some redundant
text.

Change-Id: I6ede8084aebbd49c5a27aa427ef9782d99a347aa
Reviewed-on: http://gerrit.openafs.org/10270
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>