added note about Oracle DB connection charset

close #11387

framework/db/Connection.php

@@ -238,10 +238,13 @@ class Connection extends Component
     /**
      * @var string the charset used for database connection. The property is only used
      * for MySQL, PostgreSQL and CUBRID databases. Defaults to null, meaning using default charset
-     * as specified by the database.
+     * as configured by the database.
      *
-     * Note that if you're using GBK or BIG5 then it's highly recommended to
-     * specify charset via DSN like 'mysql:dbname=mydatabase;host=127.0.0.1;charset=GBK;'.
+     * For Oracle Database, the charset must be specified in the [[dsn]], for example for UTF-8 by appending `;charset=UTF-8`
+     * to the DSN string.
+     *
+     * The same applies for if you're using GBK or BIG5 charset with MySQL, then it's highly recommended to
+     * specify charset via [[dsn]] like `'mysql:dbname=mydatabase;host=127.0.0.1;charset=GBK;'`.
      */
     public $charset;
     /**