[AWS] Clarify the role of "Default AWS Region"

The AWS package allows for collecting logs & metrics from a rich selection of AWS services.

All integrations in the package share the global settings (access/secret keys, role, region, etc) to access the AWS API using the AWS SDK.

The "Default AWS Region" is essential in one of these settings. Its behavior is documented in the package UI (see screenshot below), but it does not have a dedicated space in the AWS package [docs](https://www.elastic.co/guide/en/integrations/current/aws.html).

The use of `us-east-1` as the default region creates authentication issues that are hard to troubleshoot when users deploy the Elastic Agent on AWS partitions like `aws-us-gov` or `aws-cn`.

We need to:

- Review the current package information on the UI and docs.
- Identify the gaps we need to address.
- Make changes to improve the OOTB user experience, including users from `aws-us-gov`, `aws-cn`, and other existing partitions.