Fix documentation of last commit

Fix documentation of last commit

* lisp/xwidget.el (xwidget-webkit-cookie-file): Don't use "path"
for file names in the doc string.  Improve wording and markup of
the doc string.

* src/xwidget.c (Fxwidget_webkit_set_cookie_storage_file):
* doc/lispref/display.texi (Xwidgets): Don't use "path" for file
names.

* etc/NEWS: Improve the wording of the entry about
'xwidget-webkit-cookie-file'.