added missing GettextMessageSource phpdoc

framework/yii/i18n/GettextMessageSource.php

@@ -9,6 +9,23 @@ namespace yii\i18n;
 
 use Yii;
 
+/**
+ * GettextMessageSource represents a message source that is based on GNU Gettext.
+ *
+ * Each GettextMessageSource instance represents the message tranlations
+ * for a single domain. And each message category represents a message context
+ * in Gettext. Translated messages are stored as either a MO or PO file,
+ * depending on the [[useMoFile]] property value.
+ *
+ * All translations are saved under the [[basePath]] directory.
+ *
+ * Translations in one language are kept as MO or PO files under an individual
+ * subdirectory whose name is the language ID. The file name is specified via
+ * [[catalog]] property, which defaults to 'messages'.
+ *
+ * @author Qiang Xue <qiang.xue@gmail.com>
+ * @since 2.0
+ */
 class GettextMessageSource extends MessageSource
 {
 	const MO_FILE_EXT = '.mo';