[3.7] bpo-35054: Add more index entries for symbols. (GH-10064). (GH-10120)

(cherry picked from commit ddb961d2abe5d5fde76d85b21a77e4e91e0043ad)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/library/constants.rst b/Doc/library/constants.rst
index 78f1619..634ff00 100644
--- a/Doc/library/constants.rst
+++ b/Doc/library/constants.rst
@@ -51,10 +51,11 @@
       See :exc:`NotImplementedError` for details on when to use it.
 
 
+.. index:: single: ...; ellipsis literal
 .. data:: Ellipsis
 
-   The same as ``...``.  Special value used mostly in conjunction with extended
-   slicing syntax for user-defined container data types.
+   The same as the ellipsis literal "``...``".  Special value used mostly in conjunction
+   with extended slicing syntax for user-defined container data types.
 
 
 .. data:: __debug__