Fix inconsistency in dynamic application log levels

As of present, there are two ways of changing log levels in Runtime:

1. JMX API
2. Admin API

However, there are some inconsistencies between these two ways;

1. JMX API allows OFF level; but Admin API does not. If the user tries to set the logger's level to OFF, Kafka responds with INVALID_CONFIG (40) error.
2. JMX API converts unsupported log level to DEBUG; but Admin API throws an error. The documentation does not state this difference in semantics.

To fix these inconsistencies, we have to:

1. Add OFF level to LogLevelConfig.
2. Add documentation on different semantics between JMX and Admin API.