Experiments AI Summary limitations

Bug Description

Using the summarize feature on an experiment is hitting some regular limitations in response including

• I'm encountering the same technical issue with the experiment summary tool. Let me search for this experiment to get its details.
• ...but I encountered a technical issue when trying to generate an automated summary - it appears there may be a configuration problem with how the experiment's goal is set up.
• Unfortunately, I can't access the actual statistical results due to a technical issue with the experiment summary tool. To see the full results including variant performance and significance, I recommend viewing the experiment directly.

How to reproduce

1. Navigate to an experiment
2. click the Summarize button

Additional context

Reported by a customer and verified in a couple of other running experiments

The request would be to document any limitations and improve new capabilities for experiments here https://posthog.com/docs/posthog-ai/tools#limitations

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables or https://eu.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]

Start with the PostHog AI tools documentation and its limitations section linked in the issue. Compare the reported experiment-summary responses with the current documentation, then document confirmed limitations and clarify which requested capabilities remain unsupported. Done means the page accurately reflects the verified behavior and the reproduction steps are addressed.