Improved answer posted by Grant Winney, please upvote his answer.

To avoid warnings and make the <para> work as desired, use this in derived class:

/// <summary>Time To Live.
/// <para>Also stops ticking after living time is longer than TTL.</para>
/// </summary>
new public float TTL
{
    get { return base.TTL; }
    set { base.TTL = value; }
}