docs: explain nondeterministic row ordering without ORDER BY (#7373)

docs/source/user-guide/faq.md

@@ -63,3 +63,16 @@ when DataFusion might be suitable or unsuitable for your needs:
   database system. Like DataFusion it is also written in Rust and
   utilizes the Apache Arrow memory model, but unlike DataFusion it
   targets end-users rather than developers of other database systems.
+
+## Why do my query results come back in a different order between runs?
+
+DataFusion only guarantees row order when the query includes an `ORDER BY`
+clause.
+
+Without `ORDER BY`, operators such as joins, `GROUP BY`, `UNION`, and
+parallel file scans may emit the same rows in a different order across runs.
+This is normal for a parallel execution engine and does not mean the query
+result is incorrect.
+
+If you need stable output ordering, add an explicit `ORDER BY` clause to the
+outermost query.

docs/source/user-guide/sql/select.md

@@ -86,7 +86,13 @@ SELECT a FROM table WHERE a > 10
 
 ## JOIN clause
 
-DataFusion supports `INNER JOIN`, `LEFT OUTER JOIN`, `RIGHT OUTER JOIN`, `FULL OUTER JOIN`, `NATURAL JOIN`, `CROSS JOIN`, `LEFT SEMI JOIN`, `RIGHT SEMI JOIN`, `LEFT ANTI JOIN`, and `RIGHT ANTI JOIN`.
+DataFusion supports `INNER JOIN`, `LEFT OUTER JOIN`, `RIGHT OUTER JOIN`,
+`FULL OUTER JOIN`, `NATURAL JOIN`, `CROSS JOIN`, `LEFT SEMI JOIN`,
+`RIGHT SEMI JOIN`, `LEFT ANTI JOIN`, and `RIGHT ANTI JOIN`.
+
+Unless you add an `ORDER BY` clause, joins do not guarantee the order of
+the returned rows. DataFusion executes queries in parallel, so the same
+join query may produce the same rows in a different order across runs.
 
 The following examples are based on this table:
 
@@ -246,6 +252,10 @@ Example:
 SELECT a, b, MAX(c) FROM table GROUP BY a, b
 ```
 
+`GROUP BY` determines how rows are grouped for aggregation, but it does not
+determine the order of the output rows. If you need a stable row order, add
+an `ORDER BY` clause to the outer query.
+
 Some aggregation functions accept optional ordering requirement, such as `ARRAY_AGG`. If a requirement is given,
 aggregation is calculated in the order of the requirement.
 
@@ -294,6 +304,11 @@ FROM table2
 Orders the results by the referenced expression. By default it uses ascending order (`ASC`).
 This order can be changed to descending by adding `DESC` after the order-by expressions.
 
+Without `ORDER BY`, DataFusion does not guarantee the order of result rows.
+This is especially important for queries involving joins, `GROUP BY`,
+`UNION`, or parallel file scans, where rows may be returned in a different
+order between runs even when the data itself has not changed.
+
 Examples:
 
 ```sql