Radix documents the 11/12 APCA guarantee against a step 2 background
only, not step 1. Correct the claim and the example (which used
bg-sand-1), add the precise Lc 60 / Lc 90 targets and a source link,
and note that step 1 is safe in practice but not part of the stated
guarantee.
Signed-off-by: Émile Ré <emile@probo.com>