eww: Use browse-url-with-browser-kind in eww-browse-with-external-browser

eww: Use browse-url-with-browser-kind in eww-browse-with-external-browser

Guarantee that an external browser is used by EWW if
`browse-url-secondary-browser-function' is set to
`eww-browse-url'.
* lisp/net/eww.el (eww-browse-with-external-browser): Use
`browse-url-secondary-browser-function' only if it is an
external browser, otherwise fall back to
`browse-url-with-browser-kind'.
(eww-follow-link): Use `eww-browse-with-external-browser' if the
EXTERNAL prefix argument is non-nil.  Improve docstring.
* lisp/net/browse-url.el (browse-url-secondary-browser-function):
Update docstring.  (Bug#74730)

(cherry picked from commit a3610381ff65490da33cd1870dd3dfd491bff06f)