Add a note to QCheck{,2}.Gen.graft_corners

Author: jmid

src/core/QCheck.mli

@@ -325,6 +325,12 @@ module Gen : sig
   val graft_corners : 'a t -> 'a list -> unit -> 'a t
   (** [graft_corners gen l ()] makes a new generator that enumerates
       the corner cases in [l] and then behaves like [g].
+
+      Note that [graft_corners gen l ()] is stateful, meaning that once the
+      elements of [l] have been emitted, subsequent calls will not reproduce
+      them. It is therefore recommended that separate tests each use a fresh
+      generator.
+
       @since 0.6 *)
 
   val int_pos_corners : int list

src/core/QCheck2.mli

@@ -634,6 +634,11 @@ module Gen : sig
       Does not shrink if the test fails on a grafted value.
       Shrinks towards [gen] otherwise.
 
+      Note that [graft_corners gen l ()] is stateful, meaning that once the
+      elements of [l] have been emitted, subsequent calls will not reproduce
+      them. It is therefore recommended that separate tests each use a fresh
+      generator.
+
       @since 0.6 *)
 
   val int_pos_corners : int list