doc: Add status_code to Completed signal(s)

doc: Add status_code to Completed signal(s)

It seems we have not provided any means to return if operations
have succeeded or not.

This commit introduces `status_code` field, which allows service
to return status, if/as needed.

Surely, adding signals <Operation>Failed would be better from
design perspective, and might be considered in future if time
allows (dbus signals are private interface and subject to change
without notice).

Change-Id: Ib77681597c6633c62ae0936040f26633a9bdad05