I stumbled across a comment that began with "XXX". It was clearly meant as
a "TODO" comment. This changes all such occurrences. However, a few "XXX"
comments are ambiguous and a better prefix might be "WARNING". The "TODO"
prefix at least ensures someone, eventually, looks into the situation
and either rewords the comment or fixes the problem. This change means
everyone can assume searching for "// TODO" will find all such comments
rather than requiring they also know to search for "// XXX".
