add docs around assuming role for techdocs storage

docs/features/techdocs/using-cloud-storage.md

@@ -149,7 +149,7 @@ If the environment variables
 - `AWS_REGION`
 
 are set and can be used to access the bucket you created in step 2, they will be
-used by the AWS SDK v3 Node.js client for authentication.
+used by the AWS SDK v2 Node.js client for authentication.
 [Refer to the official documentation.](https://docs.aws.amazon.com/sdk-for-javascript/v3/developer-guide/loading-node-credentials-environment.html)
 
 If the environment variables are missing, the AWS SDK tries to read the
@@ -181,7 +181,7 @@ techdocs:
 ```
 
 Refer to the
-[official AWS documentation for obtaining the credentials](https://docs.aws.amazon.com/sdk-for-javascript/v3/developer-guide/getting-your-credentials.html).
+[official AWS documentation for obtaining the credentials](https://docs.aws.amazon.com/sdk-for-javascript/v2/developer-guide/getting-your-credentials.html).
 
 Note: If you are using Amazon EC2 instance to deploy Backstage, you do not need
 to obtain the access keys separately. They can be made available in the
@@ -189,6 +189,27 @@ environment automatically by defining appropriate IAM role with access to the
 bucket. Read more
 [here](https://docs.aws.amazon.com/general/latest/gr/aws-access-keys-best-practices.html#use-roles).
 
+**3c. Authentication using an assumed role** Users with multiple AWS accounts
+may want to use a role for S3 storage that is in a different AWS account. Using
+the `roleArn` parameter as seen below, you can instruct the TechDocs publisher
+to assume a role before accessing S3.
+
+```yaml
+techdocs:
+  publisher:
+    type: 'awsS3'
+    awsS3:
+      bucketName: 'name-of-techdocs-storage-bucket'
+      region:
+        $env: AWS_REGION
+      credentials:
+        roleArn: arn:aws:iam::123456789012:role/my-backstage-role
+```
+
+Note: Assuming a role requires that primary credentials are already configured
+at `AWS.config.credentials`. Read more about assuming roles
+[here](https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html)
+
 **4. That's it!**
 
 Your Backstage app is now ready to use AWS S3 for TechDocs, to store and read