clarify that base path can be the empty string

[Rich-Harris]

Apparently some people get confused by this (e.g. #1329)

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[Conduitry]

This broke tests for me in https://github.com/sveltejs/kit/tree/master/packages/kit/src/core/config/index.spec.js (which I would expect because it changed an error message, and they're now looking for the wrong one), but CI is still passing, which is bugging me.

[mrkishi]

Apparently the tests haven't been running since #4707, the green/neutral checks are Netlify/Cloudflare/Vercel checks.

I think include should be an array instead, #4727 seems to be running the tests.

[Conduitry]

Ah, thank you!