[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/gettext.rst b/Doc/library/gettext.rst
index 407853c..93748a2 100644
--- a/Doc/library/gettext.rst
+++ b/Doc/library/gettext.rst
@@ -61,6 +61,7 @@
    *domain*, which is returned.
 
 
+.. index:: single: _; gettext
 .. function:: gettext(message)
 
    Return the localized translation of *message*, based on the current global