Additional documentation on permitted scalars and nested parameter configuration

I had a lot of trouble getting my roles parameter to pass, but I came from a position where I didn't understand Strong Parameters very well. After digging in and getting that understanding, I thought an update to the Devise documentation on the difference between Strong Parameters permitted scalar types and more complex types might help future generations avoid the long winding research I undertook.

So, please, see if you think this would help at all