DocumentDB Autoscaling available?

Thank you. I've read that page. Does it mean that auto-scaling is available for storage but not compute?

Correct, at this time there is no built-in mechanism to scale the compute layer automatically. You could implement a solution based on Cloudwatch and Lambda, for example when the alarm threshold is reached, Lambda script will be triggered to add a new replica to the cluster (scale out). See https://aws.amazon.com/blogs/database/amazon-documentdb-with-mongodb-compatibility-read-autoscaling/ or https://github.com/theuves/docdb-autoscaling/tree/master