std:.raw::Slice doc is not precise enough

[RalfJung]

The doc of std:.raw::Slice leaves some questions unanswered: First of all, it would be useful to repeat that a slice is a "&[T]". I first though I'd look at a "[T]".

Also, looking at http://doc.rust-lang.org/std/raw/struct.Slice.html, it's not clear whether this is the actual implementation of a slice (so, all slice operations desugar to something with this type in an early compilation phase), or whether this is more of an example showing what the compiler does internally, that's not actually used when my code operates on slices. (This point also applies to the other stuff in std::raw.)