fix(navigation): Improve helm-apropos usage

- Assign `helm-apropos` to `<helm-prefix> d` instead of the default
  `<helm-prefix> a`, as that is now taken up by `helm-do-ag`.
- Use the `helpful` package to describe functions and variables.

modules/help/siren-helpful.el

@@ -14,7 +14,11 @@
   ("C-h C" . helpful-command)
   ("C-h F" . helpful-function)
   (:map emacs-lisp-mode-map
-        ("C-c C-d" . helpful-at-point)))
+        ("C-c C-d" . helpful-at-point))
+
+  :custom
+  (helm-describe-function-function 'helpful-function)
+  (helm-describe-variable-function 'helpful-variable))
 
 (provide 'siren-helpful)
 ;;; siren-helpful.el ends here