Small modification to documentation around cube constructor

Hello,

this is a very small modification. I wanted to try put it into a pull request on its own as its my first attempt at doing this.  As it's small it hopefully won't waste a huge amount of anyone's time.

I have a couple of reservations about what I've done...

1. there is a comment in the code that talks about a temporary interface to Cube - and I'm not sure if the behaviour I've documented will be changed in the near future (and so maybe wasn't documented before on purpose)

2. I'm not sure whether I should have referenced the numpy documentation - this may be considered as revealing too much of the implementation detail (I don't know).

Jamie