xfs: expanding delayed logging design with background material

I wrote up a description of how transactions, space reservations and
relogging work together in response to a question for background
material on the delayed logging design. Add this to the existing
document for ease of future reference.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>