improve performance of query callback test

the behavior being tested did not depend on any data actually
being indexed, so that was removed

additional documentation was added to clarify what this test
was testing, since it was not clear and we did not remember

index_test.go

@@ -1896,29 +1896,15 @@ func TestSearchQueryCallback(t *testing.T) {
 		}
 	}()
 
-	elements := []string{"air", "water", "fire", "earth"}
-	b := index.NewBatch()
-	for j := 0; j < 10000; j++ {
-		err = b.Index(fmt.Sprintf("%d", j),
-			map[string]interface{}{"name": elements[j%len(elements)]})
-		if err != nil {
-			t.Fatal(err)
-		}
-	}
-	err = index.Batch(b)
-	if err != nil {
-		t.Fatal(err)
-	}
-
 	query := NewTermQuery("water")
 	req := NewSearchRequest(query)
 
 	expErr := fmt.Errorf("MEM_LIMIT_EXCEEDED")
 	f := func(size uint64) error {
-		if size > 1000 {
-			return expErr
-		}
-		return nil
+		// the intended usage of this callback is to see the estimated
+		// memory usage before executing, and possibly abort early
+		// in this test we simulate returning such an error
+		return expErr
 	}
 
 	ctx := context.WithValue(context.Background(), SearchQueryStartCallbackKey,