Add a URL to support a TODO comment.

Summary: Provide a link to a "resolved" Xdebug issue to support a TODO comment. Further information can be found at http://derickrethans.nl/dead-code.html.

Test Plan: Read the article.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9696