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 Similar to the (out of date) ntb_tool description.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>

Documentation/ntb.txt

@@ -200,6 +200,33 @@ Debugfs Files:
 	This file is used to read and write peer scratchpads.  See
 	*spad* for details.
 
+NTB MSI Test Client (ntb\_msi\_test)
+------------------------------------
+
+The MSI test client serves to test and debug the MSI library which
+allows for passing MSI interrupts across NTB memory windows. The
+test client is interacted with through the debugfs filesystem:
+
+* *debugfs*/ntb\_tool/*hw*/
+	A directory in debugfs will be created for each
+	NTB device probed by the tool.  This directory is shortened to *hw*
+	below.
+* *hw*/port
+	This file describes the local port number
+* *hw*/irq*_occurrences
+	One occurrences file exists for each interrupt and, when read,
+	returns the number of times the interrupt has been triggered.
+* *hw*/peer*/port
+	This file describes the port number for each peer
+* *hw*/peer*/count
+	This file describes the number of interrupts that can be
+	triggered on each peer
+* *hw*/peer*/trigger
+	Writing an interrupt number (any number less than the value
+	specified in count) will trigger the interrupt on the
+	specified peer. That peer's interrupt's occurrence file
+	should be incremented.
+
 NTB Hardware Drivers
 ====================