Document that channel name is added as [@metadata]

The [code](https://github.com/logstash-plugins/logstash-input-redis/blob/71d3ee5d75bd42458685e0ffd0520727d9ccaa21/lib/logstash/inputs/redis.rb#L163) adds the channel name as [@metadata][redis_channel] if the datatype option is channel or pattern_channel. If the datatype is list then channel will be nil and the metadata will not be set. None of this is mentioned in the documentation. It should be.