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