[docs-update] Update docs for form composer guidance

[github-actions]

Summary

• Updates the form docs to call out the composer-style textarea exception to the usual no-React-state guidance for Field.Root
• Adds a worked example showing a controlled Textarea inside Form with a sr-only Field.Label, Field.Error, and server rejections routed through errors

Changesets

• .changeset/clean-moons-invite.md — documents the public guidance around Form + Field for composer-style submissions

Docs updated

• docs/components/form.md — expanded the onFormSubmit guidance and added a composer-specific example covering controlled draft state, real label wiring, submit semantics, and error handling

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