Doc: added brief statement for i18n

Change-Id: Idd686f4ff9ebf876117caf7165098ce305cdc3f0
Task-number: QTBUG-42682
Reviewed-by: Martin Smith <martin.smith@digia.com>

doc/src/internationalization/i18n.qdoc

@@ -27,6 +27,7 @@
 
 /*!
     \group i18n
+    \brief How to internationalize your Qt application.
     \title Qt Classes for Internationalization
 
     See \l{Internationalization with Qt} for information on how to use these classes