bpo-35054: Add yet more index entries for symbols. (GH-10121) (GH-10171)

(cherry picked from commit 913876d824d969f8c7431e8a9d4610a9a11a786e)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Doc/library/gettext.rst b/Doc/library/gettext.rst
index 93748a2..c98c5e7 100644
--- a/Doc/library/gettext.rst
+++ b/Doc/library/gettext.rst
@@ -61,7 +61,7 @@
    *domain*, which is returned.
 
 
-.. index:: single: _; gettext
+.. index:: single: _ (underscore); gettext
 .. function:: gettext(message)
 
    Return the localized translation of *message*, based on the current global