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

(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.