fix comments

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/Optimizer.scala

@@ -539,9 +539,10 @@ object LimitPushDown extends Rule[LogicalPlan] {
     // pushdown Limit.
     case LocalLimit(exp, u: Union) =>
       LocalLimit(exp, u.copy(children = u.children.map(maybePushLocalLimit(exp, _))))
-    // Add extra limits below OUTER JOIN. For LEFT OUTER and RIGHT OUTER JOIN we push limits to
-    // the left and right sides, respectively. It's not safe to push limits below FULL OUTER
-    // JOIN in the general case without a more invasive rewrite.
+    // Add extra limits below JOIN. For LEFT OUTER and RIGHT OUTER JOIN we push limits to
+    // the left and right sides, respectively. For INNER and CROSS JOIN we push limits to
+    // both the left and right sides if join condition is empty. It's not safe to push limits
+    // below FULL OUTER JOIN in the general case without a more invasive rewrite.
     // We also need to ensure that this limit pushdown rule will not eventually introduce limits
     // on both sides if it is applied multiple times. Therefore:
     //   - If one side is already limited, stack another limit on top if the new limit is smaller.