Add docs of update/replace *_FAILED resource for developer
Add docs of update/replace *_FAILED resource for developer

After we implemented the blueprint
https://blueprints.launchpad.net/heat/+spec/custom-update-replace-for-failed-resources
it is good to add docs for developer about this.

Change-Id: I36ebe184497ecac0b8477d6778a0121dcb8ea406

noironetworks/heat,noironetworks/heat,openstack/heat,openstack/heat