docs: explain why route id can be null

[sacrosanctic]

I couldn't get symlinks to work so I'm just winging it :/

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[benmccann]

You've edited the generated file. You need to instead update src/exports/public.d.ts and then run pnpm generate:types

[benmccann]

I think we can tweak this to be a bit more concise and avoid the word "fails" which might indicate some error occurred rather than expected behavior

Suggested change

	* The ID of the current route - e.g. for `src/routes/blog/[slug]`, it would be `/blog/[slug]`. It is null when the router fails to match a route.
	* The ID of the current route - e.g. for `src/routes/blog/[slug]`, it would be `/blog/[slug]`. The ID is `null` when no route is matched.

[changeset-bot]

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-13581-svelte.vercel.app/

[Rich-Harris]

accepted ben's suggestion and applied the changes to public.d.ts. thanks!