Minor additions to i18n docs

docs/guide/i18n.md

@@ -39,14 +39,17 @@ Later you can easily change it in runtime:
 Basic message translation
 -------------------------
 
-### Strings translation
-
-Yii basic message translation that works without additional PHP extension.
+Yii basic message translation in its basic variant works without additional PHP extension. What it does is finding a
+translation of the message from source language into targer language. Message itself is specified as the first
+`\Yii::t` method parameter:
 
 ```php
 echo \Yii::t('app', 'This is a string to translate!');
 ```
 
+Yii tries to load approprite translation from one of the message sources defined via `i18n` component configuration.
+
+TBD: https://github.com/yiisoft/yii2/issues/930
 
 ### Named placeholders