Adjust the comment explaining why this version of the code is correct

The previous comment was clear enough for me to finally understand what the bug
here is and why this change fixes it.  Hopefully this comment is clear enough that
other people might be able to understand it too.

refs #4935


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/unjelly-newstyle-reference-cycles-4935@43324 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb