NTB: switchtec_ntb: Update switchtec documentation with notes for NTB

The switchtec_ntb driver has a couple requirements on the switchec's
hardware configuration so we add these notes to the documentation.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Stephen Bates <sbates@raithlin.com>
Reviewed-by: Kurt Schwemmer <kurt.schwemmer@microsemi.com>
Acked-by: Allen Hubbe <Allen.Hubbe@dell.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>