docs: fix wording, note SvelteKit 1.x behavior

[dummdidumm]

• note that error/redirect throws, and document SvelteKit 1.x behavior
• fix promise behavior description and note SvelteKit 1.x behavior

[changeset-bot]

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[benmccann]

This might be a good opportunity to note the changes from #11258, which Pablo said weren't mentioned yet

[Rich-Harris]

We don't need to be stingy with PRs, we can document the query params stuff separately