NTB: Describe the ntb_msi_test client in the documentation.

Add a blurb in Documentation/ntb.txt to describe the ntb_msi_test tool's
debugfs interface. Similar to the (out of date) ntb_tool description.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>