xfs: Delayed logging design documentation

Document the design of the delayed logging implementation. This
includes assumptions made, dead ends followed, the reasoning behind
the structuring of the code, the layout of various structures, how
things fit together, traps and pit-falls avoided, etc. This is all
too much to document in the code itself, so do it in a separate
file.
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Alex Elder <aelder@sgi.com>