lib/storage: improve the message of the tooManyTimeseries error (#6893)

### Describe Your Changes

This is a follow-up for #6836. Per @valyala's
[comment](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6836#discussion_r1730291704),
the error message does not reflect which flag needs to be adjusted.

### Checklist

The following checks are **mandatory**:

- [x ] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: Artem Fetishev <wwctrsrx@gmail.com>
(cherry picked from commit 4df243d530)

lib/storage/index_db.go

@@ -2385,7 +2385,8 @@ func (is *indexSearch) searchMetricIDs(qt *querytracer.Tracer, tfss []*TagFilter
 
 func errTooManyTimeseries(maxMetrics int) error {
 	return fmt.Errorf("the number of matching timeseries exceeds %d; "+
-		"either narrow down the search or increase -search.max* command-line flag values at vmselect; "+
+		"either narrow down the search or increase -search.max* command-line flag values at vmselect "+
+		"(the most likely limit is -search.maxUniqueTimeseries); "+
 		"see https://docs.victoriametrics.com/#resource-usage-limits", maxMetrics)
 }