Merge forward.

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/clarify_reactor_doscstring-5951-2@37905 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb
Apply ticket-5951-2-docs.patch from wirehead.

Refs: #5951

git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/branches/clarify_reactor_doscstring-5951-2@37906 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb