clarify Set.diff documentation (#2119)

fixes [MPR#7868](https://caml.inria.fr/mantis/view.php?id=7868)

.mailmap

@@ -117,6 +117,7 @@ Miod Vallat <miod@mantis>
 Christoph Spiel <cspiel@mantis>
 Joris Giovannangeli <joris@mantis>
 Wilfred Hughes <wilfred@fb.com> <wilfred@mantis>
+John Skaller <skaller@mantis>
 
 # These contributors prefer to be referred to pseudonymously
 whitequark <whitequark@whitequark.org>

Changes

@@ -130,6 +130,9 @@ Working version
   (Laurent Thévenoux, review by Alain Frisch, Jacques-Henri Jourdan,
   Xavier Leroy)
 
+- GPR#2119: clarify the documentation of Set.diff
+  (Gabriel Scherer, suggestion by John Skaller)
+
 ### Other libraries:
 
 - GPR#1061: Add ?follow parameter to Unix.link. This allows hardlinking

stdlib/set.mli

@@ -103,7 +103,8 @@ module type S =
         @since 4.08.0 *)
 
     val diff: t -> t -> t
-    (** Set difference. *)
+    (** Set difference: [diff s1 s2] contains the elements of [s1]
+       that are not in [s2]. *)
 
     val compare: t -> t -> int
     (** Total ordering between sets. Can be used as the ordering function