Turbopack: Document new advanced `turbopack.rule.*.condition` config options (#83246)
~I'm looking for review/feedback, but **DO NOT MERGE UNTIL NEXT 16.0 IS RELEASED.** These APIs are not available on 15.5, and merging it now would be confusing to users. If you want to try it out, use a recent canary release.~ I've been informed that docs are not currently tracking canary, so this is safe to merge.
Documented the new configuration syntax added in https://github.com/vercel/next.js/pull/82857 that should ship with Next 16.0.
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