xen: document balloon driver sysfs files

Add ABI documentation for the balloon driver's sysfs files.
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Daniel Kiper <dkiper@net-space.pl>
[v2: Added comments from Daniel]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/ABI/stable/sysfs-devices-system-xen_memory

+What:		/sys/devices/system/xen_memory/xen_memory0/max_retry_count
+Date:		May 2011
+KernelVersion:	2.6.39
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		The maximum number of times the balloon driver will
+		attempt to increase the balloon before giving up.  See
+		also 'retry_count' below.
+		A value of zero means retry forever and is the default one.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/max_schedule_delay
+Date:		May 2011
+KernelVersion:	2.6.39
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		The limit that 'schedule_delay' (see below) will be
+		increased to. The default value is 32 seconds.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/retry_count
+Date:		May 2011
+KernelVersion:	2.6.39
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		The current number of times that the balloon driver
+		has attempted to increase the size of the balloon.
+		The default value is one. With max_retry_count being
+		zero (unlimited), this means that the driver will attempt
+		to retry with a 'schedule_delay' delay.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/schedule_delay
+Date:		May 2011
+KernelVersion:	2.6.39
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		The time (in seconds) to wait between attempts to
+		increase the balloon.  Each time the balloon cannot be
+		increased, 'schedule_delay' is increased (until
+		'max_schedule_delay' is reached at which point it
+		will use the max value).
+
+What:		/sys/devices/system/xen_memory/xen_memory0/target
+Date:		April 2008
+KernelVersion:	2.6.26
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		The target number of pages to adjust this domain's
+		memory reservation to.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/target_kb
+Date:		April 2008
+KernelVersion:	2.6.26
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		As target above, except the value is in KiB.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/info/current_kb
+Date:		April 2008
+KernelVersion:	2.6.26
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		Current size (in KiB) of this domain's memory
+		reservation.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/info/high_kb
+Date:		April 2008
+KernelVersion:	2.6.26
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		Amount (in KiB) of high memory in the balloon.
+
+What:		/sys/devices/system/xen_memory/xen_memory0/info/low_kb
+Date:		April 2008
+KernelVersion:	2.6.26
+Contact:	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
+Description:
+		Amount (in KiB) of low (or normal) memory in the
+		balloon.