They use the C++ style:
1) you can put you comments from "/*" to "*/"
2) you can start comments from "//" until the end of line
For example:
/* RT example
more line, this is a comment
*/
{object
...
model { sphere // comment
until the end of line
...
}
}
.
Remember that if you use with some automatic programs which work on .rt file you will lose these comments!