remove some unimportant or incorrect information from the _unjelly_instance method docstring; reformat the warnings.warn call

refs #7653


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/deprecate-jelly-instance-support-7653@43241 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb