Small documentation changes

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.
Fixed documentation for finalize->drop change
Removing the rename warning

Now that the finalize->drop rename has happened, this hopefully
shouldn't end up changing again.
auto merge of #7549 : sfackler/rust/docs, r=msullivan

I'm leaving the Sized kind undocumented since it isn't fully implemented
yet.
Add VecRef to std::vec

This adds a `VecRef` that can hold either a ~[T] or a &'static [T]
Implement a StringRef

This is the string equivalent of VecRef. I had some trouble with
lifetimes trying to implement StrSlice, so I've left it for now,
iterating on strings isn't as common and the .as_slice() method
merely means you need to add an extra method call.