docs recommend invoking config-changed, but no example

[dann frazier]

The docs say "If the install or start hooks don't directly need to deal with options they can simply invoke the config-changed hook." but, when writing my first charm, I've found that the config-changed hook seems to already run during deploy on options set with a "--config local.yaml" - no explicit call was necessary. I think this should be clarified in the docs.