doc: Document fs listquota 2TB partition limit

We have previously documented that volumes over 2TB can result in
inaccuracies, but this documentation does not say how the 'partition'
field in "fs listquota" can be inaccurate. It is confusing to see a
usage of 0% for a partition that you know is being used, so try to
briefly explain in what way this field is inaccurate.

The reason we _under_-report the partition usage is that the
fileserver actually gives back PartBlocksAvail and PartMaxBlocks (not
"blocks used" and "blocks total"). So 1TB used and 4TB total is
truncated to 2TB and given back as 2TB free and 2TB total. One we hit
3TB used we'll report it as 1TB free 2TB total (50%) when the actual
usage is 75%.

Change-Id: I0b3de04ef2bd6cd32fdcb1a82cbac58d5d621e5b
Reviewed-on: http://gerrit.openafs.org/11245
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>