Mention the new type system in typespecs doc (#14188)

Author: sabiwara

lib/elixir/pages/references/typespecs.md

@@ -1,7 +1,12 @@
 # Typespecs reference
 
-Elixir comes with a notation for declaring types and specifications. This document is a
-reference into their uses and syntax.
+> #### Typespecs are not set-theoretic types {: .warning}
+>
+> Elixir is in the process of implementing its
+> [own type system](./gradual-set-theoretic-types.md) based on set-theoretic types.
+> Typespecs, which are described in the following document, are a distinct notation
+> for declaring types and specifications based on Erlang.
+> Typespecs may be phased out as the set-theoretic type effort moves forward.
 
 Elixir is a dynamically typed language, and as such, type specifications are never used by the compiler to optimize or modify code. Still, using type specifications is useful because:
 
@@ -48,6 +53,13 @@ The syntax Elixir provides for type specifications is similar to [the one in Erl
 
 The notation to represent the union of types is the pipe `|`. For example, the typespec `type :: atom() | pid() | tuple()` creates a type `type` that can be either an `atom`, a `pid`, or a `tuple`. This is usually called a [sum type](https://en.wikipedia.org/wiki/Tagged_union) in other languages
 
+> #### Differences with set-theoretic types {: .warning}
+>
+> While they do share some similarities, the types below do not map one-to-one
+> to the new types from the set theoretic type system.
+> For example, there is no plan to support subsets of the `integer()` type such
+> as positive, ranges or literals.
+
 ### Basic types
 
     type ::