Fix the documentation parts of a recent commit

Fix the documentation parts of a recent commit

* lisp/international/mule-util.el (truncate-string-ellipsis): Doc
fix.

* doc/lispref/display.texi (Size of Displayed Text): Improve
wording and accuracy of the documentation of
'truncate-string-to-width'.  Document the function
'truncate-string-ellipsis'.

* etc/NEWS: Improve the wording of the entry for
'truncate-string-to-width'.