doc: add design rationale comments for transparency settings

This PR adds documentation comments to the key transparency-related
definitions: `TransparencyMode`, `ReducibilityStatus`,
`instance_reducible`, `backward.isDefEq.respectTransparency`,
`backward.whnf.reducibleClassField`, `canUnfoldDefault`, and
`isDefEqArgs`. The comments explain the "try-hard" vs "speculative"
isDefEq distinction, the transparency hierarchy, why instance diamonds
require unfolding, why implicit arguments received special treatment,
and the motivation for the v4.29 changes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>