docs: document virtio-balloon stats

Document statistics added in commits

    commit a0d06486
    Author: Denis V. Lunev <den@openvz.org>
    Date:   Wed Feb 24 10:50:48 2016 +0300

        virtio-balloon: add 'available' counter

and

    commit bf1e7140
    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: NTomáš Golembiovský <tgolembi@redhat.com>
Reviewed-by: NJonathan Helman <jonathan.helman@oracle.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

docs/virtio-balloon-stats.txt

@@ -32,6 +32,8 @@ which will return a dictionary containing:
       - stat-minor-faults
       - stat-free-memory
       - stat-total-memory
+      - stat-available-memory
+      - stat-disk-caches
 
   o A key named last-update, which contains the last stats update
     timestamp in seconds. Since this timestamp is generated by the host,