Openapi emitter should respect return type variant doc on response

[timotheeguerin]

import "@typespec/http";

using TypeSpec.Http;

model PetCreated {
  @statusCode code: 201;
}

model PetAccepted {
  @statusCode code: 202;
}

union CreatedResponses {
  @doc("Pet Created") created: PetCreated,
  @doc("Pet Updated") accepted: PetAccepted,
}

op created(): CreatedResponses;

here Pet Created and Pet Updated doc is lost.

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