afs: Add some comments explaining 'needvalid'

afs: Add some comments explaining 'needvalid'

Add some comments to try and help explain the meaning of the
'needvalid' parameter to GetDSlot, added in
1a672914ab050811c99b6307c657630ab9b5c8ee.

Reviewed-on: http://gerrit.openafs.org/7429
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 6f6c799c4ba4d03c6c381738b2db8aafe741ba5e)

Change-Id: If7d37d427f96f3607b6c3aea2487396666e41a03
Reviewed-on: http://gerrit.openafs.org/7943
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>