docs(migrate): qualify Goose zero-target behavior

migrate/lifecycle/lifecycle_test.go

@@ -199,7 +199,7 @@ func TestCommandsSQLite(t *testing.T) {
 			t.Fatal(calls)
 		}
 	}
-	// Zero is valid syntax; Goose UpTo rejects it after setup, unlike DownTo(0).
+	// Zero is valid syntax; Goose UpTo rejects it after setup when migrations are pending.
 	code, err := run(t, []string{"up-to", "0"}, []lifecycle.Module{module}, resolver, &out)
 	if code != 1 || err == nil || !strings.Contains(err.Error(), "greater than 0") {
 		t.Fatalf("zero UpTo: code=%d err=%v", code, err)