Move this warning out of the dict unjellier method and into the class docstring

The warning applies to more than just dictionaries.
Also expand it to discuss the consequences of this issue.

refs #4935


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/unjelly-newstyle-reference-cycles-4935@43325 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb