s390/cpum_cf: Add kernel message exaplanations

The CPU Measurement facility for counters and counter set
rework adds a few new kernel messages to the system log.
Add an explanation for some of these.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>