Add fixme about least_fixpoint_persistent lemmas

iris/bi/lib/relations.v

@@ -154,6 +154,8 @@ Section general.
     ∀ x y, Affine (bi_rtc R x y).
   Proof. intros. apply least_fixpoint_affine; apply _. Qed.
 
+  (* FIXME: It would be nicer to use the least_fixpoint_persistent lemmas,
+            but they seem to weak. *)
   Global Instance bi_rtc_persistent :
     (∀ x y, Persistent (R x y)) →
     ∀ x y, Persistent (bi_rtc R x y).
@@ -263,6 +265,8 @@ Section general.
     ∀ x y, Absorbing (bi_tc R x y).
   Proof. intros. apply least_fixpoint_absorbing; apply _. Qed.
 
+  (* FIXME: It would be nicer to use the least_fixpoint_persistent lemmas,
+            but they seem to weak. *)
   Global Instance bi_tc_persistent :
     (∀ x y, Persistent (R x y)) →
     ∀ x y, Persistent (bi_tc R x y).