Don't show "Right-hand side" for data fam-instances

Don't show "Right-hand side" for data fam-instances

GHC's RHS for a @data instance@ is the synthetic R: TyCon (e.g.
@R:CrateInt@), and after the data-family R: rewrite it's the
abstract @FamilyApp Family [args]@ form — structurally equal to the
LHS. Labelling the panel row "Right-hand side: Crate Int" was
misleading because it just repeated what the heading already said.
Replace with a short "Data instance — the right-hand side is a
synthetic GHC-internal data constructor and isn't shown." note for
data fam-instances; type fam-instances keep the real RHS line.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>