DRIVER-671 Document aws_region config

[dkropachev]

Jira: https://scylladb.atlassian.net/browse/DRIVER-671

Closes #9

Summary:

• document the AWS SDK reason for Config::aws_region next to the public field
• clarify that Alternator routing ignores the region and the default is only a placeholder
• point users to set a deployment or Scylla Cloud region for meaningful SDK observability

Testing:

• make check