docs: add UNION (dedup) example to SQL set operations section

Only UNION ALL was shown in the example. Added UNION without ALL to
clarify that deduplicating set union is also supported.

Author: teamchong

docs/src/content/docs/sql.mdx

@@ -115,7 +115,8 @@ Simple CTEs (no aggregation, no sort+limit) are inlined — their filters merge
 ### Set operations
 
 ```sql
-SELECT * FROM orders_2025 UNION ALL SELECT * FROM orders_2026
+SELECT * FROM orders_2025 UNION ALL SELECT * FROM orders_2026   -- keeps duplicates
+SELECT * FROM orders_2025 UNION SELECT * FROM orders_2026       -- deduplicates
 SELECT region FROM orders INTERSECT SELECT region FROM users
 SELECT id FROM orders EXCEPT SELECT id FROM refunds
 ```