Use '…' for ellipsis in truncate-string-to-width by default (bug#41250)

Use '…' for ellipsis in truncate-string-to-width by default (bug#41250)

* lisp/international/mule-util.el (truncate-string-ellipsis):
Change the default value to nil.
(truncate-string-ellipsis): New function.
(truncate-string-to-width): Use the value returned from the
function 'truncate-string-ellipsis'.

* lisp/tab-bar.el (tab-bar-tab-name-truncated):
* lisp/tab-line.el (tab-line-tab-name-ellipsis):
Take advantage of the improvement of the ellipsis default value
in truncate-string-to-width and truncate-string-ellipsis.

* doc/lispref/display.texi (Size of Displayed Text):
Improve description of truncate-string-ellipsis.