Update document of Kafka Source: Enable Dynamic Partition Discovery by Default in Kafka Source

Based on Flip 288,  dynamic partition discovery is enabled by Default in Kafka Source  now. some corresponding document in Chinese and English should be modified:

• "Partition discovery is disabled by default. You need to explicitly set the partition discovery interval to enable this feature" in https://nightlies.apache.org/flink/flink-docs-master/docs/connectors/datastream/kafka/

• scan.topic-partition-discovery.interval is (none) in https://nightlies.apache.org/flink/flink-docs-master/docs/connectors/table/kafka/