(describe-symbol-backends): Fix addition of the "type" backend

That backend was added from `cl-extra.el` with no autoload, so
(describe-symbol `advice) failed to show the info about
the `advice` type unless `cl-extra.el` had been loaded beforehand.
`C-h o RET advice RET` worked by accident because the completion
table uses `cl-some` which is autoloaded from `cl-extra.el`.

* lisp/help-mode.el (describe-symbol-backends): Add the "type" backend.
* lisp/emacs-lisp/cl-extra.el (describe-symbol-backends): Don't add the
"type" backend here.

lisp/emacs-lisp/cl-extra.el

@@ -711,13 +711,6 @@ PROPLIST is a list of the sort returned by `symbol-plist'.
 (eval-when-compile (require 'cl-macs))  ;Explicitly, for cl--find-class.
 (require 'help-mode)
 
-;; FIXME: We could go crazy and add another entry so describe-symbol can be
-;; used with the slot names of CL structs (and/or EIEIO objects).
-(add-to-list 'describe-symbol-backends
-             `(nil ,#'cl-find-class ,#'cl-describe-type)
-             ;; Document the `cons` function before the `cons` type.
-             t)
-
 (defconst cl--typedef-regexp
   (concat "(" (regexp-opt '("defclass" "defstruct" "cl-defstruct"
                             "cl-deftype" "deftype"))

lisp/help-mode.el

@@ -545,6 +545,9 @@ it does not already exist."
         (or (and (boundp symbol) (not (keywordp symbol)))
             (get symbol 'variable-documentation)))
      ,#'describe-variable)
+    ;; FIXME: We could go crazy and add another entry so describe-symbol can be
+    ;; used with the slot names of CL structs (and/or EIEIO objects).
+    ("type" ,#'cl-find-class ,#'cl-describe-type)
     ("face" ,#'facep ,(lambda (s _b _f) (describe-face s))))
   "List of providers of information about symbols.
 Each element has the form (NAME TESTFUN DESCFUN) where: