normalize: document the transforms as spec-bug workarounds to retire

Reframe the normalizer's doc comment: both transforms exist only to compensate for bugs in the upstream OpenAPI document, and the goal is to delete the whole command once the spec is fixed at the source and ogen generates straight from core.openapi.json. Each transform now names the exact spec fix that retires it — non-nullable arrays for the type-union collapse, real per-operation success codes (201/204/200) for the 2XX collapse — and notes that fixing the response codes also removes the *…StatusCode wrapper the 2XX range forces.

Comment-only; regenerating produces an identical core.gen.json.