kvno invocation correction, language cleanup, afs/cell principal preferred

kvno invocation correction, language cleanup, afs/cell principal preferred

Properly show kvno command syntax, add information about preferring
'afs/cell' for the principal over 'afs', and changed "noted this down"
to "made note of"

Change-Id: I56fca2e34ce18634a155c8eb3b21760f2ee990c6
Reviewed-on: http://gerrit.openafs.org/4740
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>