[Fargate] [docs?]: Clarify Fargate limits

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
I'd like some clarification around the limits for ECS on Fargate. In [the documented limits](https://docs.aws.amazon.com/general/latest/gr/ecs-service.html#limits_ecs) we see a clear limit for ECS on EC2:
> Tasks using the EC2 launch type per service (the desired count) : 1.000

Even though no relevant Fargate limits are specified, I can't seem to be able to scale 1 Service using Fargate Spot to anything above 1.000 tasks, no matter what I do. Adding a second service works fine, but that service is also limited to 1.000 tasks.

This is the error I get: `Error: InvalidParameterException: Desired count exceeds limit.`

Does the limit apply to Fargate too (not just EC2)? Is this a limit that can be increased? I'd love some more details about what the Fargate limits are. Are the limits for Fargate Spot different? What about ECS on Fargate vs EKS on Fargate?

**Which service(s) is this request for?**
ECS on Fargate

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
I am doing some benchmarks :)

**Are you currently working around this issue?**
Multiple services with the same definition seem to work fine.

Start with the ECS service limits page linked in the issue and reproduce the Fargate scaling case that returns “Desired count exceeds limit.” Compare the documented EC2, ECS on Fargate, Fargate Spot, and EKS on Fargate limits, then update the relevant AWS documentation so the applicable limits and increase options are explicit.

Written by the indexing model from the issue text.