thread safety up

### Problem:

[Thread safety](https://github.com/aws/aws-encryption-sdk-python?tab=readme-ov-file#thread-safety) section contains link to the [resources doc](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/resources.html#multithreading-or-multiprocessing-with-resources) but aws-encryption-sdk-python does not use boto3 resources and use boto3 client. According to [clients doc](https://boto3.amazonaws.com/v1/documentation/api/latest/guide/clients.html#multithreading-or-multiprocessing-with-clients) it's thread-safe and could be used in a multithreaded application.

I read #560 and [https://github.com/aws/aws-encryption-sdk-python/issues/11#issuecomment-330716036](https://github.com/aws/aws-encryption-sdk-python/issues/11#issuecomment-330716036), there used irrelevant boto docs, because aws-encryption-sdk-python does not use boto3 resource, but client.

### Solution:

Review multithreaded guidelines and update doc.

Start with the Thread safety section in the README and compare its current resources link with the boto3 clients multithreading guidance cited in the issue. Update the documentation to reflect the SDK's use of boto3 clients and their thread-safety guidance; done means the links and wording are accurate.

Written by the indexing model from the issue text.