NTB: Add new Memory Windows API documentation

Since the new API slightly changes the way a typical NTB client driver
works, the documentation file needs to be appropriately updated.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>