doc: document the vldb free list

Document vldb free list in the vldb format (vldb.txt). The nextIdHash[0]
is on the free chain when the vl entry is free.

Also fix two typos in vldb.txt.

Change-Id: I5d79f55214295e029e7174ef46838afd7dc44bf1
Reviewed-on: http://gerrit.openafs.org/11597
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>