Add DRAKE_ASSERT guidance per #1935.
Add DRAKE_ASSERT guidance per #1935.

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