Document connections_max_idle_ms

[jeffwidman]

Need to document connections_max_idle_ms which was added in #1068.

Should be documented in KafkaProducer, KafkaConsumer, and KafkaClient.

As part of documenting, perhaps for the consumer should also add a mention in both request timeout and session timeout that you must always set connection timeout > request timeout > session timeout.

For context:
One of the teams I support was changing their session timeout to greater than 10 minutes for a particular use case. I realized they needed to change the request timeout, but didn't realize they needed to change the connection timeout. As a result, the broker sat idly waiting for the consumer to come back, even after they'd lowered the session timeout and restarted the consumers, because it was waiting for the old consumer (which was never coming back) using the old session timeout.