docs: adding info on task access restrictions and APIs

[marcosgvieira]

Description

I'm adding more information about APIs as we got feedback from a customer that this wasn't clear here: https://jira.camunda.com/browse/SUPPORT-21876?atlLinkOrigin=c2xhY2staW50ZWdyYXRpb258aXNzdWU%3D

When should this change go live?

This change can go live at any moment.

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer.

[akeller]

I think some of the assignees are meant to be reviewers here. @marcosgvieira can you double check?