Document PreludeArrays module

[matil019]

Some modules in the fregedoc has very nice overview. For example, frege.java.util.Regex.

I'd like to have a similar documentation for frege.prelude.PreludeArrays, especially about converting to and from lists and arrays. JArrays already has a detailed documentation on handling Java arrays in Frege, but it doesn't describe the interaction between arrays and lists e.g. how to ArrayOf s a -> ST s [a].