x86/intel_rdt: Add documentation for "info/last_cmd_status"

New file in the "info" directory helps diagnose what went wrong
when using the /sys/fs/resctrl file system

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Vikas Shivappa <vikas.shivappa@intel.com>
Cc: Boris Petkov <bp@suse.de>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Link: https://lkml.kernel.org/r/387e78e444582403c2454479e576caf5721a363f.1506382469.git.tony.luck@intel.com

Documentation/x86/intel_rdt_ui.txt

@@ -87,6 +87,17 @@ with the following files:
 			bytes) at which a previously used LLC_occupancy
 			counter can be considered for re-use.
 
+Finally, in the top level of the "info" directory there is a file
+named "last_cmd_status". This is reset with every "command" issued
+via the file system (making new directories or writing to any of the
+control files). If the command was successful, it will read as "ok".
+If the command failed, it will provide more information that can be
+conveyed in the error returns from file operations. E.g.
+
+	# echo L3:0=f7 > schemata
+	bash: echo: write error: Invalid argument
+	# cat info/last_cmd_status
+	mask f7 has non-consecutive 1-bits
 
 Resource alloc and monitor groups
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