ksqlDB: limit in count messages

wrt https://kafka-tutorials.confluent.io/how-to-count-messages-on-a-kafka-topic/ksql.html

> by default, ksqlDB emits table results every 2 seconds (based on default commit.interval.ms)

> Maybe a little extra explanation would be helpful. While Without this, ksqlDB will continue to write any changes in the count to the screen as new messages arrive is totally true, it’s also not guaranteed that the first row will contain the complete count of existing records, if the topic is large

Someone "followed this tutorial to count records in a topic, and was surprised that the result was much lower than expected."

Thanks for reporting the issue @mikebin

cc: @rmoff