doc: state klog.krb is obsolete

doc: state klog.krb is obsolete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.

Reviewed-on: http://gerrit.openafs.org/10270
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 8fc86de33e95efc5cdcd73131d97a2afd1ad935a)

Change-Id: I1fbd3920495a855b34e8547e90a941d36cafdfdf
Reviewed-on: http://gerrit.openafs.org/10314
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>