doc: Fix incorrect UUID index entry in function documentation.

Previously, the UUID functions documentation defined the "UUID" index entry
to link to the UUID data type page, even though that entry already exists there.
Instead, the UUID functions page should define its own index entry linking
to itself.

This commit updates the UUID index entry in the UUID functions documentation
to point to the correct section, improving navigation and avoiding duplication.

Back-patch to all supported versions.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/f33e0493-5773-4296-87c5-7ce459054cfe@oss.nttdata.com
Backpatch-through: 13

doc/src/sgml/func.sgml

@@ -13423,7 +13423,7 @@ CREATE TYPE rainbow AS ENUM ('red', 'orange', 'yellow', 'green', 'blue', 'purple
  <sect1 id="functions-uuid">
   <title>UUID Functions</title>
 
-  <indexterm zone="datatype-uuid">
+  <indexterm zone="functions-uuid">
    <primary>UUID</primary>
    <secondary>generating</secondary>
   </indexterm>