User guide cube broadcasting documentation.

Added user guide documentation coverage for cube broadcasting support during cube arithmetic.
Cheers @pelson 

I wasn't aiming for showing off ... just trying to making it crystal that juggling the dimensions **plus** removing a dimension (particularly not a leading or trailing one) still just works.

If you think it's a step too far, well I'm happy to take the last example out. Personally I thought it was kinda neat, but I would ...