Document reasoning about aws region

[dkropachev]

Jira task: https://scylladb.atlassian.net/browse/DRIVER-673
Jira epic: https://scylladb.atlassian.net/browse/DRIVER-27

Copied from Jira epic DRIVER-27:

There is a problem that we provide bogus AWS region name to the aws client.

If client have tracing/debuging or anything else that looks at aws region it can create confusion or missbehavior.

We need to document reasoning behind bogus AWS region and a way to set it properly.

Example case: https://github.com/scylladb/alternator-load-balancing/issues/30

Migrated from GitHub issue: scylladb/alternator-load-balancing#32

[dkropachev]

Closing as completed by #44.

PR #44 added the aws_region configuration option, wires it into sync and async SDK client creation, and documents that the region is a placeholder required by the AWS SDK while Alternator routing still uses discovered Alternator endpoints.