Added documentation for TreeLists, and slightly changed treelist-empty? and treelist-length types

Author: Ryan Ficklin

typed-racket-doc/typed-racket/scribblings/reference/types.scrbl

@@ -352,6 +352,8 @@ corresponding to @racket[trest], where @racket[bound]
 @deftypeconstr[(MListof t)]{Returns the type of a homogeneous @rtech{mutable list} of @racket[t].}
 @deftypeconstr[(MPairof t u)]{Returns the type of a @rtech{Mutable pair} of @racket[t] and @racket[u].}
 
+@deftypeconstr[(TreeListof t)]{Returns the type of @rtech{treelist} of @racket[t]}
+
 @deftype[MPairTop]{Is the type of a @rtech{mutable pair} with unknown
   element types and is the supertype of all mutable pair types.
   This type typically appears in programs via the combination of

typed-racket-lib/typed-racket/base-env/base-env.rkt

@@ -1298,8 +1298,8 @@
 ;; Section 4.21 (TreeLists)
 
 [treelist (-poly (a) (->* (list) a (-treelist a)))]
-[treelist-empty? (-poly (a) (-> (-treelist a) B))]
-[treelist-length (-poly (a) (-> (-treelist a) -Index))]
+[treelist-empty? (-> (-treelist Univ) B : (-treelist (Un)))]
+[treelist-length (-> (-treelist Univ) -Index)]
 [treelist-member?
  (-poly (a)
         (cl->* ((-treelist a) a . -> . Univ)