docs: select ret null, return ret hello world (#42073)

Select returns null

return returns hello world

(so this example has always been broken)

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-->

## Summary by CodeRabbit

* **Documentation**
* Updated the database functions guide with refined code examples.
Documentation now demonstrates improved Postgres function syntax and
streamlined return mechanisms, providing developers with clearer
guidance for implementing database functions following current best
practices.

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settings.</sub>

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apps/docs/content/guides/database/functions.mdx

@@ -416,7 +416,7 @@ language plpgsql
 security definer set search_path = ''
 as $$
 begin
-  select 'hello world';
+  return 'hello world';
 end;
 $$;
 ```