docs: clarify what handleFetch affects

[eltigerchino]

This PR includes some additional server functions that handleFetch affects that are left out in the current description. Technically it also affects the event.fetch in handleFetch itself, but I'm wondering why that's a thing in the first place since it would make it recursive 😅

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13788-svelte.vercel.app/

[dummdidumm]

Let's revert it for now, a h4 is rendered super tiny on svelte.dev (maybe we should adjust that over there; not sure)

Suggested change

	#### Credentials
	**Credentials**

[Rich-Harris]

Neither makes sense — we deliberately don't use bolding anywhere, and avoid deeply nested heading hierarchies. I think we should just reword it (it doesn't really need to be a subsection). Will push something