Add documentation of cxx_inl pattern.
Add documentation of cxx_inl pattern.

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