Update documentation

[tenderlove]

The documentation for Fiddle.dlwrap and Fiddle.dlunwrap were not very accurate and pretty confusing. This commit updates the documentation so it's easier to understand what the methods do.

[tenderlove]

@zverok oops, thanks. I'll update the comments with your suggestions.

I think explaining tagged pointers is probably outside the scope of Fiddle documentation. I expect developers that are messing around with pointers to Ruby objects need to know a bit about Ruby internals, but I wanted to at least call out that not all Ruby objects passed to dlwrap will return a memory address.