docs: clarify async and callback plugin styles

[MauricioMoraisZage]

Summary

Clarifies that plugin functions should not mix callback and async styles.

This adds a short documentation note explaining that async plugin functions should not use the done callback, and shows separate examples for callback-style and async-style plugins.

Related issue

Refs: #221

[Fdawgs]

Already covered by Fastify's docs: https://fastify.dev/docs/latest/Reference/Hooks/?_highlight=mix#respond-to-a-request-from-a-hook

[MauricioMoraisZage]

Already covered by Fastify's docs: https://fastify.dev/docs/latest/Reference/Hooks/?_highlight=mix#respond-to-a-request-from-a-hook

Thanks for the review.

I understand that this behavior is already covered in the Fastify documentation. Would it be useful to replace this section with a short cross-reference to the existing Fastify docs, or would you prefer me to close this PR?

[Fdawgs]

Not needed at all, as the FST_ERR_PLUGIN_INVALID_ASYNC_HANDLER will be thrown. See https://fastify.dev/docs/latest/Reference/Errors/?_highlight=mix#fastify-error-codes.

The mentioned issue pre-dates the error code mentioned.

Thanks though.