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

(cherry picked from commit 6c48bf2d9e)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Doc/library/pprint.rst

@@ -30,6 +30,8 @@ The :mod:`pprint` module defines one class:
 .. First the implementation class:
 
 
+.. index:: single: ...; placeholder
+
 .. class:: PrettyPrinter(indent=1, width=80, depth=None, stream=None, *, \
                          compact=False)
 

Doc/library/reprlib.rst

@@ -42,6 +42,9 @@ In addition to size-limiting tools, the module also provides a decorator for
 detecting recursive calls to :meth:`__repr__` and substituting a placeholder
 string instead.
 
+
+.. index:: single: ...; placeholder
+
 .. decorator:: recursive_repr(fillvalue="...")
 
    Decorator for :meth:`__repr__` methods to detect recursive calls within the

Doc/library/textwrap.rst

@@ -268,6 +268,8 @@ hyphenated words; only then will long words be broken if necessary, unless
       .. versionadded:: 3.4
 
 
+   .. index:: single: ...; placeholder
+
    .. attribute:: placeholder
 
       (default: ``' [...]'``) String that will appear at the end of the output