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

diff --git a/Doc/library/constants.rst b/Doc/library/constants.rst
index a698ff8..5017159 100644
--- a/Doc/library/constants.rst
+++ b/Doc/library/constants.rst
@@ -51,9 +51,10 @@
       See :exc:`NotImplementedError` for details on when to use it.
 
 
+.. index:: single: ...; ellipsis literal
 .. data:: Ellipsis
 
-   The same as the ellipsis literal "...".  Special value used mostly in conjunction
+   The same as the ellipsis literal "``...``".  Special value used mostly in conjunction
    with extended slicing syntax for user-defined container data types.