in_kafka: Add async_commit description

Signed-off-by: HaChanHo <[email protected]>

Author: HaChanho

pipeline/inputs/kafka.md

@@ -15,6 +15,7 @@ This plugin uses the official [librdkafka C library](https://github.com/edenhill
 | poll\_ms | Kafka brokers polling interval in milliseconds. | 500 |
 | Buffer\_Max\_Size | Specify the maximum size of buffer per cycle to poll kafka messages from subscribed topics. To increase throughput, specify larger size. | 4M |
 | poll\_ms | Kafka brokers polling interval in milliseconds. | 500 |
+| async_commit | Enable Kafka commit operation asynchronously. This option takes a boolean value: True/False. | False |
 | rdkafka.{property} | `{property}` can be any [librdkafka properties](https://github.com/edenhill/librdkafka/blob/master/CONFIGURATION.md) |  |
 
 ## Getting Started