[aws][docs] Update max session duration in sts_assume_role documentation(#41765)

Update to reflect assume role updated durations as of March 2018. 
https://aws.amazon.com/blogs/security/enable-federated-api-access-to-your-aws-resources-for-up-to-12-hours-using-iam-roles/

+label: docsite_pr

lib/ansible/modules/cloud/amazon/sts_assume_role.py

@@ -36,8 +36,9 @@ options:
       - Supplemental policy to use in addition to assumed role's policies.
   duration_seconds:
     description:
-      - The duration, in seconds, of the role session. The value can range from 900 seconds (15 minutes) to 3600 seconds (1 hour).
-        By default, the value is set to 3600 seconds.
+      - The duration, in seconds, of the role session. The value can range from 900 seconds (15 minutes) to 43200 seconds (12 hours).
+        The max dependis on the IAM role's sessions duration setting.
+        By default, the value is set to 3600 seconds.s
   external_id:
     description:
       - A unique identifier that is used by third parties to assume a role in their customers' accounts.