docs: update form prop description

[ethanalvizo]

Validation behavior is not defaulting to aria as expected. Updated docs to reflect it. Can create a ticket if needed to merge this through.

✅ Pull Request Checklist:

• [x] Included link to corresponding React Spectrum GitHub Issue.
• [x] Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• [x] Filled out test instructions.
• [x] Updated documentation (if it already exists for this component).
• [x] Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[reidbarber]

Thanks! You'll just need to sign the CLA, then close and re-open this for it the pass CI checks.