docs: kafka_max_block_size default value is max_insert_block_size, not max_block_size

[ByronWang]

default max_insert_block_size

size_t StorageKafka::getMaxBlockSize() const
{
    return kafka_settings->kafka_max_block_size.changed
        ? kafka_settings->kafka_max_block_size.value
        : (getContext()->getSettingsRef().max_insert_block_size.value / num_consumers);
}

[ByronWang]

kafka_max_block_size default value is max_insert_block_size, not max_block_size.

[SpencerTorres]

Linking the actual code for future reference: https://github.com/ClickHouse/ClickHouse/blob/bebed0598d968e01fd806c22ffaa622df5b50778/src/Storages/Kafka/StorageKafka.cpp#L863-L868