(help-fns-function-description-header): Print functions' type

(help-fns-function-description-header): Print functions' type

Instead of choosing English words to describe the kind of function,
use the actual type of the function object (from `cl-type-of`)
directly, and make it a button to display info about that type.

* lisp/help-fns.el (help-fns-function-description-header): Use the
function's type name in the description instead of "prose".
Use `insert` instead of `princ`, so as to preserve the text-properties
of the button.

* lisp/emacs-lisp/cl-extra.el (cl-help-type): Move to `help-mode.el`
and rename to `help-type`.
(cl--describe-class): Adjust accordingly.

* lisp/help-mode.el (help-type): New type, moved and renamed from
`cl-extra.el`.