Clarify ordering syntax rationale

lib/Db/Driver.php

@@ -75,8 +75,8 @@ interface Driver {
      * - "nocase": the name of a general-purpose case-insensitive collation sequence
      * - "like": the case-insensitive LIKE operator
      * - "integer": the integer type to use for explicit casts
-     * - "asc": ascending sort order
-     * - "desc": descending sort order
+     * - "asc": ascending sort order when dealing with nulls
+     * - "desc": descending sort order when dealing with nulls
      */
     public function sqlToken(string $token): string;
 

tests/cases/Db/BaseDriver.php

@@ -394,7 +394,7 @@ abstract class BaseDriver extends \JKingWeb\Arsse\Test\AbstractTest {
         $this->assertSame("Z", $this->drv->query("SELECT 'Z' collate $nocase")->getValue());
         $this->assertSame("Z", $this->drv->query("SELECT 'Z' where 'Z' $like 'z'")->getValue());
         $this->assertEquals(1, $this->drv->query("SELECT CAST((1=1) as $integer)")->getValue());
-        $this->assertEquals([null, 1], array_column($this->drv->query("SELECT 1 as t union select null as t order by t $asc")->getAll(), "t"));
-        $this->assertEquals([1, null], array_column($this->drv->query("SELECT 1 as t union select null as t order by t $desc")->getAll(), "t"));
+        $this->assertEquals([null, 1, 2], array_column($this->drv->query("SELECT 1 as t union select null as t union select 2 as t order by t $asc")->getAll(), "t"));
+        $this->assertEquals([2, 1, null], array_column($this->drv->query("SELECT 1 as t union select null as t union select 2 as t order by t $desc")->getAll(), "t"));
     }
 }