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.

Inizia dalla sezione Thread safety del README e confronta il suo attuale link alle risorse con le indicazioni sul multithreading di boto3 clients citate nell’issue. Aggiorna la documentazione in modo da riflettere l’uso di boto3 clients da parte dell’SDK e le relative indicazioni sulla thread safety; il lavoro è completato quando i link e la formulazione sono accurati.

Scritto dal modello di indicizzazione a partire dal testo della issue.