Document the real behavior of notify_on_state_change
Document the real behavior of notify_on_state_change

The documentation of the notify_on_state_change config param was
incomplete and misleading. The code is untouched since
2012 so the document is updated to reflect the current behavior.

Change-Id: I51d0deffc4f42ff2722a8d21aa6b8c8008c62723
Closes-Bug: #1535254
(cherry picked from commit e86604fb6579e2333ab9ab5a6926ef84d201152a)

vmturbo/nova,vmturbo/nova,vmturbo/nova,vmturbo/nova