When calling help on undocumented DataTypes, show more information.

Right now, when calling help on undocumented types, all that is returned is the type name itself.  This patch adds slightly more information:

```julia
julia> IOStream
IOStream

julia> help(IOStream)
DataType: IOStream
  fields: (:handle,:ios,:name)

julia> DataType
DataType

julia> help(DataType)
DataType: DataType
  fields: (:fptr,:env,:code,:name,:super,:parameters,:names,:types,:ctor_factory,:instance,:size,:abstract,:mutable,:pointerfree)
```