doc: Add index entry for MERGE SQL command.

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

Author: MasaoFujii

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>