NTB: switchtec_ntb: Update switchtec documentation with prerequisites for NTB

The ntb_hw_switchtec driver has requirements on kernel configuration so add
these notes to the documentation and also clean up a few other sentences in
the documentation.

Signed-off-by: Wesley Yung <wesley.yung@microchip.com>
Signed-off-by: Kelvin Cao <kelvin.cao@microchip.com>
Signed-off-by: Wesley Sheng <wesley.sheng@microchip.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>