docs: add example for chaperone-treelist

pkgs/racket-doc/scribblings/reference/treelists.scrbl

@@ -488,7 +488,30 @@ of @racket[prepend-proc] is a chaperone, then that chaperone's
 @racket[append-proc] keep returning chaperones, it is possible that
 no progress will be made.
 
-}
+@examples[
+#:eval the-eval
+(chaperone-treelist
+ (treelist 1 "a" 'apple)
+ #:state 'ignored-state
+ #:ref (λ (tl pos v state) (code:comment @#,elem{or @racket[#f]})
+         v)
+ #:set (λ (tl pos v state)
+         (values v state))
+ #:insert (λ (tl pos v state)
+            (values v state))
+ #:delete (λ (tl pos state)
+            state)
+ #:take (λ (tl pos state)
+          state)
+ #:drop (λ (tl pos state)
+          state)
+ #:append2 (λ (tl other state other-state) (code:comment @#,elem{or @racket[#f]})
+             (values other state))
+ #:append (λ (tl other state)
+            (values other state))
+ #:prepend (λ (other tl state)
+             (values other state)))
+ ]}
 
 @defproc[(treelist-chaperone-state [tl treelist?]
                                    [state-key any/c]