Add a backwards compatibility note.

Author: ZeroIntensity

Doc/c-api/intro.rst

@@ -188,7 +188,8 @@ complete listing.
    variable and then pass it.
 
    *type* is the type of the expression. In modern versions of Python, this is
-   unused. Historically, *type* was used to cast *integer*.
+   unused and only kept for backwards compatibility. Historically, *type* was
+   used to cast *integer*.
 
    .. versionchanged:: 3.1