[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/struct.rst b/Doc/library/struct.rst
index f10fbe4..3b15158 100644
--- a/Doc/library/struct.rst
+++ b/Doc/library/struct.rst
@@ -117,6 +117,13 @@
 order, and properly aligned by skipping pad bytes if necessary (according to the
 rules used by the C compiler).
 
+.. index::
+   single: @; in struct format strings
+   single: =; in struct format strings
+   single: <; in struct format strings
+   single: >; in struct format strings
+   single: !; in struct format strings
+
 Alternatively, the first character of the format string can be used to indicate
 the byte order, size and alignment of the packed data, according to the
 following table: