bpo-9842: Add references for using "..." as a placeholder to the index. (GH-10330)

(cherry picked from commit 6c48bf2d9e1e18dfbfa35f7582ddd32f11f75129)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/library/textwrap.rst b/Doc/library/textwrap.rst
index 438007d..d254466 100644
--- a/Doc/library/textwrap.rst
+++ b/Doc/library/textwrap.rst
@@ -268,6 +268,8 @@
       .. versionadded:: 3.4
 
 
+   .. index:: single: ...; placeholder
+
    .. attribute:: placeholder
 
       (default: ``' [...]'``) String that will appear at the end of the output