Add some documentation

lean/Spa/Language/Semantics.lean

@@ -51,6 +51,8 @@ inductive EvalBasicStmt : Env → BasicStmt → Env → Type
   | assign (ρ : Env) (x : String) (e : Expr) (v : Value) :
       EvalExpr ρ e v → EvalBasicStmt ρ (.assign x e) ((x, v) :: ρ)
 
+/-- Inference rules for evaluating a basic-statement-or-nothing,
+    which is the current representation of CFGs nodes. -/
 inductive EvalBasicStmtOpt : Env → Option BasicStmt → Env → Type
   | none {ρ : Env} : EvalBasicStmtOpt ρ Option.none ρ
   | some {ρ₁ ρ₂ : Env} {bs : BasicStmt} :