doc: Add index entry for MERGE SQL command.

MasaoFujii · MasaoFujii · commit 7c2d6f8d3400 · 2022-07-01T14:19:52.000+09:00
Back-patch to v15, where MERGE command was supported. Author: Fujii Masao Reviewed-by: Alvaro Herrera Discussion: https://postgr.es/m/4e7a4b38-c34a-ce2c-c71e-832d2b768b29@oss.nttdata.com
diff --git a/doc/src/sgml/ref/merge.sgml b/doc/src/sgml/ref/merge.sgml
@@ -4,6 +4,9 @@ PostgreSQL documentation
 -->
 
 <refentry id="sql-merge">
+ <indexterm zone="sql-merge">
+  <primary>MERGE</primary>
+ </indexterm>
 
  <refmeta>
   <refentrytitle>MERGE</refentrytitle>
