docs: clarify use:enhance compatibility with method="POST"

[garysassano]

This PR aims to improve the documentation by explicitly stating that use:enhance is only compatible with forms using method="POST". Despite the fact that using use:enhance with other methods should throw an error, this limitation isn't clearly communicated in the current documentation.

This lack of clarity has led to misuse in several prominent projects, including:

• storybook
• sveltekit-superforms

By updating the documentation, we can help prevent similar misunderstandings and improve the developer experience when working with SvelteKit forms.

---

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[eltigerchino]

Thank you! (and sorry this took so long)