bpo-29981: Add examples and update index for set, dict, and generator comprehensions'(GH-20272)


Co-authored-by: Rémi Lapeyre <remi.lapeyre@henki.fr>
(cherry picked from commit 2d55aa9e37c9c84f4f6a8135d0326da0bcd8f38b)

Co-authored-by: Florian Dahlitz <f2dahlitz@freenet.de>
diff --git a/Doc/reference/expressions.rst b/Doc/reference/expressions.rst
index f1f19c3..938a973 100644
--- a/Doc/reference/expressions.rst
+++ b/Doc/reference/expressions.rst
@@ -162,6 +162,8 @@
 Displays for lists, sets and dictionaries
 -----------------------------------------
 
+.. index:: single: comprehensions
+
 For constructing a list, a set or a dictionary Python provides special syntax
 called "displays", each of them in two flavors:
 
@@ -260,6 +262,7 @@
 
 .. index::
    pair: set; display
+   pair: set; comprehensions
    object: set
    single: {} (curly brackets); set expression
    single: , (comma); expression list
@@ -287,6 +290,7 @@
 
 .. index::
    pair: dictionary; display
+   pair: dictionary; comprehensions
    key, datum, key/datum pair
    object: dictionary
    single: {} (curly brackets); dictionary expression