docs: Adjust docs for snippet typing to be more explicit

[snide]

I found the Snippet docs a little opaque in regard to typing. I think in this case it's best to be explicit with the typing, rather than use a generic, because the shape of the example is consistent throughout the page.

As an aside, I might even suggest an "Advanced patterns" section with Snippets that shows how snippet parameters can be spread to a render since that is extremely common in Svelte components in practice. I'm happy to construct an example like that if needed in a second PR.

This is only my second PR against the docs, so please let me know if I've made any stylistic or pattern errors.

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Playground

pnpm add https://pkg.pr.new/svelte@15167

[dummdidumm]

I tend to agree that a generic is not necessarily needed in a lot of cases, but it would be good to have the generic mentioned somewhere, maybe as a sentence afterwards ("For more generic reusable components, you could even use generic types to declare the relationship between them", and then another snippet).