thread safety up

[mastak]

Problem:

Thread safety section contains link to the resources doc but aws-encryption-sdk-python does not use boto3 resources and use boto3 client. According to clients doc it's thread-safe and could be used in a multithreaded application.

I read #560 and #11 (comment), there used irrelevant boto docs, because aws-encryption-sdk-python does not use boto3 resource, but client.

Solution:

Review multithreaded guidelines and update doc.