[ECS/Fargate] [request]: Document on required Linux caps for ECS Exec

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**Tell us about your request**

Give minimal [Linux capabilities](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ecs-taskdefinition-kernelcapabilities.html) to an ECS task while keeping ECS Exec working.

**Which service(s) is this request for?**

Fargate, ECS.

**Are you currently working around this issue?**

The default permissions works but I want to drop as much Linux capabilities as possible.

**Additional context**

aws-containers/amazon-ecs-exec-checker#21

No target documentation file or test is named. Start by reviewing the ECS Exec and Fargate documentation alongside the linked aws-containers/amazon-ecs-exec-checker#21 issue; done means the documentation explains which minimal Linux capabilities are required for ECS Exec and how to configure them.

Written by the indexing model from the issue text.