Finalized the new box function.

Ha!  I lied.  I mean to say that I *think* I've finalized it.  I don't
actually know yet how to get source code out of my `dtx` file.  Since
it is `l3doc`, it's a bit different.

I'm actually committing and *pushing* so that I can figure out how to
get rid of the 'this variable is documented on page ??' at the end of
every undocumented internal function.