Add DocBook documentation for the block tracepoints.

This patch adds a simple description of the various block tracepoints
available in the kernel.

Signed-off-by: William Cohen <wcohen@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>