highlighting the default rates param (#2845)

dev-docs/modules/currency.md

@@ -181,8 +181,9 @@ pbjs.setConfig({
     }
 });
 {% endhighlight %}
-Note that the `defaultRates` attribute is optional, but recommended in case
-there's an issue loading the currency file.
+
+{: .alert.alert-warning :}
+Note that the `defaultRates` attribute is optional, but recommended in case there's an issue loading the currency file.
 
 In this example, the publisher is providing their own `conversionRateFile`:
 {% highlight js %}
@@ -248,7 +249,7 @@ a currency object that may contain several parameters:
 | granularityMultiplier | `decimal` | How much to scale the price granularity calculations. Defaults to 1. | 108 |
 | conversionRateFile | `URL` | Optional path to a file containing currency conversion data. See below for the format. Prebid.org hosts a file as described in the next section. | `https://example.com/rates.json` |
 | rates | object | This optional argument allows you to specify the rates with a JSON object, subverting the need for the conversionRateFile parameter.  If this argument is specified, the conversion rate file will not be loaded. | { 'USD': { 'CNY': 6.8842, 'GBP': 0.7798, 'JPY': 110.49 } } |
-| defaultRates | `object` | An optional parameter that defines a default rate that can be used if the currency file cannot be loaded. This option isn't used when the `rates` parameter is supplied. | { 'USD': { 'GPB': 0.75 }} |
+| defaultRates | `object` | An optional **but highly recommended** parameter that defines a default rate that can be used if the currency file cannot be loaded. This option isn't used when the `rates` parameter is supplied. | { 'USD': { 'GPB': 0.75 }} |
 | bidderCurrencyDefault | `object` | This is an optional argument to provide publishers a way to define which currency is used by a particular bidder. This option was provided as a transition until such a time that most bidder adapters define currency on bid response and is kept for legacy 0.x integrations. | { "bidderXYZ": "GBP" } |
 
 ## Currency Rate Conversion File