docs: document virtio-balloon stats

docs: document virtio-balloon stats

Document statistics added in commits

    commit a0d06486b445985b8d128df172daefbae205bffd
    Author: Denis V. Lunev <den@openvz.org>
    Date:   Wed Feb 24 10:50:48 2016 +0300

        virtio-balloon: add 'available' counter

and

    commit bf1e7140ef0b3a149860ab9f05b36665133238f6
    Author: Tomáš Golembiovský <tgolembi@redhat.com>
    Date:   Tue Dec 5 13:14:46 2017 +0100

        virtio-balloon: include statistics of disk/file caches

Signed-off-by: Tomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: Jonathan Helman <jonathan.helman@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>