docs: Expand on cleanup behaviour for submission handlers

[Nessworthy]

Cover use cases for both using a submission handler that returns a promise, and for a one which does not.

Add FAQ entry about submission handler cleanup.

Following submission handling logic from https://github.com/jaredpalmer/formik/blob/main/packages/formik/src/Formik.tsx#L754-L782

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Latest commit: 2e597c8

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Name	Status	Preview	Comments	Updated (UTC)
formik-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 9, 2023 6:42pm

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[Nessworthy]

Please squash if merged, do not immortalise my formatting mistakes 😂

[quantizor]

Thanks :)