[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/sys.rst b/Doc/library/sys.rst
index 33a7b69..358b118 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1044,6 +1044,8 @@
    .. index::
       single: interpreter prompts
       single: prompts, interpreter
+      single: >>>; interpreter prompt
+      single: ...; interpreter prompt
 
    Strings specifying the primary and secondary prompt of the interpreter.  These
    are only defined if the interpreter is in interactive mode.  Their initial