Improvement the Javadoc for SpecifiedOffsetsInitializer and TimestampOffsetsInitializer.

As described in FLIP-288, Current JavaDoc does not fully explain the behavior of OffsetsInitializers. When the partition does not meet the condition, there will be a different offset strategy. This may lead to misunderstandings in the design and usage.

Add to SpecifiedOffsetsInitializer: "Use Specified offset for specified partitions while use commit offset or Earliest for unspecified partitions. Specified partition offset should be less than the latest offset, otherwise it will start from the earliest."

Add to TimestampOffsetsInitializer:Initialize the offsets based on a timestamp. If the message meeting the requirement of the timestamp have not been produced to Kafka yet, just use the latest offset.