Improve documentation of KafkaConsumer's group.id parameter

With KIP-289 and `GroupCoordinator.isValidGroupId(...)` method, we can conclude that for KafkaConsumer:
1. For consumers using the group management mechanism, `group.id` must be non-null and non-empty string
2. `group.id` can be set to empty string only in assign mode

https://kafka.apache.org/documentation/#consumerconfigs_group.id
So I suggest improving the parameter documentation as follows:

A unique string that identifies the consumer group this consumer belongs to. This property is required if the consumer uses either the group management functionality by using subscribe(topic) or the Kafka-based offset management strategy.

->

A unique string that identifies the consumer group this consumer belongs to. This property is required if the consumer uses either the group management functionality by using subscribe(topic) or the Kafka-based offset management strategy, the value must be a non-null and non-empty string. `group.id` can be set to an empty string only the consumer uses assign(topic).