whatsnew: textwrap.shorten.

Also add the missing TextWrapper.shorten method doc.
diff --git a/Doc/library/textwrap.rst b/Doc/library/textwrap.rst
index 1ba42a3..0188729 100644
--- a/Doc/library/textwrap.rst
+++ b/Doc/library/textwrap.rst
@@ -282,3 +282,13 @@
 
       Wraps the single paragraph in *text*, and returns a single string
       containing the wrapped paragraph.
+
+
+   .. method:: shorten(text, width=70, *, placeholder=" [...]")
+
+      Collapse and truncate the given text to fit in the given width.
+      The text first has its whitespace collapsed.  If it then fits in
+      the *width*, it is returned unchanged.  Otherwise, as many words
+      as possible are joined and then the *placeholder* is appended.
+
+      .. versionadded:: 3.4