Clarification of behaviour when enable.auto.commit is set to false (#333)

* Clarification of behaviour when enable.auto.commit is set to false

the behaviour for enable.auto.commit set to false was missing. adding the same to the doc

* Grouping the cases together

* Placing the common line about behaviour before the options.

Author: amitavmohanty01

docs/index.asciidoc

@@ -220,9 +220,11 @@ This will add a field named `kafka` to the logstash event containing the followi
   * Value type is <<string,string>>
   * Default value is `"true"`
 
-If true, periodically commit to Kafka the offsets of messages already returned by the consumer. 
 This committed offset will be used when the process fails as the position from
 which the consumption will begin.
+If true, periodically commit to Kafka the offsets of messages already returned by
+the consumer. If value is `false` however, the offset is committed every time the
+consumer fetches the data from the topic.
 
 [id="plugins-{type}s-{plugin}-exclude_internal_topics"]
 ===== `exclude_internal_topics`