Add comment to wp_done and slightly simplify it.

heap_lang/wp_tactics.v

@@ -9,7 +9,7 @@ Ltac wp_bind K :=
   | _ => etrans; [|fast_by apply (wp_bind K)]; simpl
   end.
 
-(* TODO: Do something better here *)
+(* Solves side-conditions generated by the wp tactics *)
 Ltac wp_done :=
   match goal with
   | |- Closed _ _ => solve_closed