add more information to problem description for hexadecimal_encoding

I personally thought the documentation for this problem was too sparse. The problem basically said, "here's the buffer dox, gl!" which I believe would be too difficult for a newcomer, especially as I had to look at the solution to figure out what was meant.

I feel as though this section doesn't need a description of how to convert from decimal to hex representation, so much as a reason why you would ever need to do it.

We might considering writing another exercise, that goes before this one, where the student experiments with decimal and hexadecimal conversions, but I feel as though they are outside the scope of bytewiser.