Issue #3436591 by arnested, adityarawat_ln, webmestre: Add description of currency setting

src/Plugin/Field/FieldFormatter/NumberFormatter.php

@@ -200,8 +200,12 @@ class NumberFormatter extends FormatterBase implements ContainerFactoryPluginInt
     if ($this->isCurrencyStyle($style)) {
       $element['currency']['#type'] = 'textfield';
       $element['currency']['#size'] = 8;
+      $element['currency']['#maxlength'] = 3;
       $element['currency']['#title'] = $this->t('Currency');
       $element['currency']['#default_value'] = $this->getSetting('currency');
+      $element['currency']['#description'] = $this->t('The 3-letter <a href=":url" target="_blank">ISO 4217 currency code</a> indicating the currency to use.', [
+        ':url' => 'https://en.wikipedia.org/wiki/ISO_4217',
+      ]);
     }
     else {
       // Force the element to render (so that the AJAX wrapper is rendered) even