SONAR-7878 Update docs about l10n

server/sonar-docs/src/pages/extend/i18n.md

@@ -26,6 +26,9 @@ Localized messages are stored in properties files:
 * The names of these files must follow the convention `<key of the plugin to translate>_<language>.properties`, for example `widgetlabs_fr.properties` or `core_fr.properties` for the core bundle. See `sonar-packaging-maven-plugin` for details on plugin key derivation.
 * Messages can accept arguments. Such entries would look like:
    * `myplugin.foo=This is a message with 2 params: the first "{0}" and the second "{1}".`
+* Messages can accept pluralization. Such entries would look like:
+   * `myplugin.foo={x, number} {x, plural, one {thing} other {things}}`
+   * Learn more about this syntax [here](https://formatjs.io/guides/message-syntax/#plural-format).
    
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