target: Add documentation on the target userspace pass-through driver

Describes the driver and its interface to make it possible for user
programs to back a LIO-exported LUN.

Thanks to Richard W. M. Jones for review, and supplementing this doc
with the first two paragraphs.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>