Add information about the pluralization option for the Composition API (#1129)

* Add information about the pluralization option for the Composition API

* Add empty line

* Add break line for 'or'

Author: domoratskii

docs/guide/essentials/pluralization.md

@@ -141,7 +141,13 @@ function customRule(choice, choicesLength, orgRule) {
 }
 ```
 
-To use the custom rules defined above, you need to set the `pluralizationRules` option of `createI18n` to a key like the following locale:
+To use the custom rules defined above, inside of `createI18n` set either:
+
+1. `pluralizationRules` (for Options API)  
+*or*
+2. `pluralRules` (for Composition API)
+
+like the the following locale:
 
 ```js
 const i18n = createI18n({