Document behavior change when sending / receiving List of messages.

[tomazfernandes]

In Spring Cloud AWS 2.x, when a List was passed as an argument to send or receive payloads, it would serialize / deserialize the list in a single message payload.

Now the framework supports sending and receiving several messages at a time using a list, which enables better overall throughput since we can parallelize and send / receive as many messages in parallel or in a batch as we'd like.

But to send / receive an actual List of POJOs in a single payload requires wrapping the list on a wrapper object.

We should properly document this for users coming form the 2.x version.

See https://github.com/awspring/spring-cloud-aws/issues/879