xfs: document btree bulk loading

Add a discussion of the btree bulk loading code, which makes it easy to
take an in-memory recordset and write it out to disk in an efficient
manner.  This also enables atomic switchover from the old to the new
structure with minimal potential for leaking the old blocks.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>