Update the shared storage design document

Update the shared storage design document to reflect the current
changes, after the implementation of the ExtStorage interface.

Signed-off-by: Constantinos Venetsanopoulos <cven@grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
[iustin@google.com: small style fixes]
Reviewed-by: Iustin Pop <iustin@google.com>