Document truncate-string-ellipsis

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

* lisp/international/mule-util.el (truncate-string-ellipsis): Doc fix.
(truncate-string-to-width): Mention in the doc string that the
default for ELLIPSIS comes from 'truncate-string-ellipsis'.

* etc/NEWS: Move the 'truncate-string-ellipsis' entry to the "Lisp
Changes" section.

doc/lispref/display.texi

@@ -1889,12 +1889,13 @@ end of the result if it falls short of @var{width}.  It is also used at
 the beginning of the result if one multi-column character in
 @var{string} extends across the column @var{start-column}.
 
+@vindex truncate-string-ellipsis
 If @var{ellipsis} is non-@code{nil}, it should be a string which will
 replace the end of @var{string} (including any padding) if it extends
 beyond @var{width}, unless the display width of @var{string} is equal
 to or less than the display width of @var{ellipsis}.  If
 @var{ellipsis} is non-@code{nil} and not a string, it stands for
-@code{"..."}.
+the value of the variable @code{truncate-string-ellipsis}.
 
 @example
 (truncate-string-to-width "\tab\t" 12 4)

etc/NEWS

@@ -166,8 +166,6 @@ have the equivalent of a primary selection.
 ** `insert-register' now leaves point after the inserted text
 when called interactively.  A prefix argument toggles this behavior.
 
-** New var `truncate-string-ellipsis' to choose how to indicate truncation.
-
 +++
 ** The new variable `term-file-aliases' replaces some files from lisp/term.
 The function `tty-run-terminal-initialization' consults this variable
@@ -1302,6 +1300,9 @@ if the terminal cannot display curved quotes.
 ** Standard output and error streams now transliterate characters via
 standard-display-table, and encode output using locale-coding-system.
 
++++
+** New var `truncate-string-ellipsis' to choose how to indicate truncation.
+
 ** Miscellaneous name change
 
 For consistency with the usual Emacs spelling, the Lisp variable

lisp/international/mule-util.el

@@ -55,7 +55,8 @@
   string)
 
 (defvar truncate-string-ellipsis "..."  ;"…"
-  "String to use to indicate truncation.")
+  "String to use to indicate truncation.
+Serves as default value of ELLIPSIS argument to `truncate-string-to-width'.")
 
 ;;;###autoload
 (defun truncate-string-to-width (str end-column