bpo-35054: Add more index entries for symbols. (GH-10064)

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