xfs: document how online fsck deals with eventual consistency

Writes to an XFS filesystem employ an eventual consistency update model
to break up complex multistep metadata updates into small chained
transactions.  This is generally good for performance and scalability
because XFS doesn't need to prepare for enormous transactions, but it
also means that online fsck must be careful not to attempt a fsck action
unless it can be shown that there are no other threads processing a
transaction chain.  This part of the design documentation covers the
thinking behind the consistency model and how scrub deals with it.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>