Expand the test method docstring, restructure to the implementation to be somewhat easier to follow (smaller expressions and more of them, with comments)

refs #4631


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/bodystructure-4631-2@36730 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Refactor the body section-handling code of `_parseFetchPairs` to try to clarify it a bit; also expand the comment explaining what the heck is going on

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/bodystructure-4631-2@36731 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb