Updated order-of-includes clarification to reflect behavior of clang-format 3.8.
Updated order-of-includes clarification to reflect behavior of clang-format 3.8.

billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake