Consider this:

/**
 * {@snippet :
 * r // @link substring=r target=Reader
 * }
 */
public class X { }

Since Reader is neither referenced by FQN nor imported, it is unknown to javadoc. In such a case, javadoc should issue an error (it doesn't) similarly to how it does it in this case:

/**
 * {@link Reader}
 */
public class X { }

error: reference not found
 * {@link Reader}
          ^

That said, in both cases, diagnostic markers (i.e. summary-detail) are embedded into generated documentation.