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>