Doc tweaks on subclassing with Candy

(As I was a bit confused about whether this was possible, as the docs didn't mention this case.)

I've also added a note about the requirement to explicitly call 'table' in Candy -- that isn't needed if your parent class is *also* a bona fide result (but it isn't in my case).

Finally, in 2nd commit, I've noted that you can load the component in parent.  This makes more sense to me, but let me know if that's a Really Bad Idea :-)