'describe-function' shows function inferred type when available

* lisp/help-fns.el (help-fns--signature): Print function type for
native compiled code.
* etc/NEWS: Add entry.

etc/NEWS

@@ -30,6 +30,11 @@ applies, and please also update docstrings as needed.
 
 * Changes in Emacs 30.1
 
+** Help
+** 'describe-function' shows function inferred type when available.
+For native compiled Lisp functions 'describe-function' prints (after
+the signature) the automatically inferred function type as well.
+
 ---
 ** New user option 'describe-bindings-outline-rules'.
 This user option controls outline visibility in the output buffer of

lisp/help-fns.el

@@ -710,7 +710,11 @@ the C sources, too."
               (high-doc (cdr high)))
           (unless (and (symbolp function)
                        (get function 'reader-construct))
-            (insert high-usage "\n"))
+            (insert high-usage "\n")
+            (when (and (featurep 'native-compile)
+                       (subr-native-elisp-p (symbol-function function))
+                       (subr-type (symbol-function function)))
+              (insert (format "\nInferred type: %s\n" (subr-type (symbol-function function))))))
           (fill-region fill-begin (point))
           high-doc)))))