Update the description of fsync and j flags in WriteConcern Javadoc so that it accurately describes the current behavior, which is:

• j: If true block until write operations have been committed to the journal. Cannot be used in combination with `fsync`. Prior to MongoDB 2.6 this option was ignored if the server was running without journaling. Starting with MongoDB 2.6 write operations will fail with an exception if this option is used when the server is running without journaling.
• fsync: If true and the server is running without journaling, blocks until the server has synced all data files to disk. If the server is running with journaling, this acts the same as the `j` option, blocking until write operations have been committed to the journal. Cannot be used in combination with `j`.