Add documentation to some modules.

lean/Spa/Interp.lean

@@ -1,7 +1,17 @@
 import Mathlib.Tactic.TypeStar
 
+/-!
+
+Interpretation to a semantic domain.
+
+This file serves to introduce the double-angle-bracket "denotation"
+notation by prodiving a class instance `Interp`, whose single
+method `interp` is what the double brackets map to. -/
+
 namespace Spa
 
+/-- A type `α` that implements this class has denotation / meaning
+   in the semantic domain `dom`. -/
 class Interp (α : Type*) (dom : outParam Type*) where
   interp : α → dom