Add return value units documentation to timevaldiff.
Add return value units documentation to timevaldiff.

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