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>

Documentation/switchtec.txt

@@ -78,3 +78,15 @@ The following IOCTLs are also supported by the device:
   between PCI Function Framework number (used by the event system)
   and Switchtec Logic Port ID and Partition number (which is more
   user friendly).
+
+
+Non-Transparent Bridge (NTB) Driver
+===================================
+
+An NTB driver is provided for the switchtec hardware in switchtec_ntb.
+Currently, it only supports switches configured with exactly 2
+partitions. It also requires the following configuration settings:
+
+* Both partitions must be able to access each other's GAS spaces.
+  Thus, the bits in the GAS Access Vector under Management Settings
+  must be set to support this.