bpo-35054: Add more index entries for symbols. (GH-10064)

diff --git a/Doc/library/configparser.rst b/Doc/library/configparser.rst
index 47d6172..fcca97a 100644
--- a/Doc/library/configparser.rst
+++ b/Doc/library/configparser.rst
@@ -295,6 +295,8 @@
 interpolation.  This means values can be preprocessed before returning them
 from ``get()`` calls.
 
+.. index:: single: %; interpolation in configuration files
+
 .. class:: BasicInterpolation()
 
    The default implementation used by :class:`ConfigParser`.  It enables
@@ -323,6 +325,8 @@
    ``%(my_dir)s/Pictures`` as the value of ``my_pictures`` and
    ``%(home_dir)s/lumberjack`` as the value of ``my_dir``.
 
+.. index:: single: $; interpolation in configuration files
+
 .. class:: ExtendedInterpolation()
 
    An alternative handler for interpolation which implements a more advanced