nimble/host: Clarify ble_gap_disc duration parameters
When zero is passed stack uses default values for duration.
@@ -1435,7 +1435,8 @@ int ble_gap_read_periodic_adv_list_size(uint8_t *per_adv_list_size);
* On expiration, the procedure ends and a
* BLE_GAP_EVENT_DISC_COMPLETE event is
* reported. Units are milliseconds. Specify
- * BLE_HS_FOREVER for no expiration.
+ * BLE_HS_FOREVER for no expiration. Specify
+ * 0 to use stack defaults.
* @param disc_params Additional arguments specifying the particulars
* of the discovery procedure.
* @param cb The callback to associate with this discovery
