Add a utility function buffer-narrowed-p, and use it.

Add a utility function buffer-narrowed-p, and use it.

* lisp/subr.el (buffer-narrowed-p): New function.

* lisp/ses.el (ses-widen):
* lisp/simple.el (count-words--buffer-message):
* lisp/net/browse-url.el (browse-url-of-buffer): Use it

* lisp/simple.el (count-words-region): Don't signal an error if there
is a non-nil prefix arg and the mark is not set.

* doc/lispref/positions.texi (Narrowing): Document buffer-narrowed-p.