Document unsafety of wasm simd intrinsics (rust-lang#1184)

Since most intrinsics are safe it likely makes sense to explicitly
document why there are a few intrinsics that are not safe. These
intrinsics are all unsafe for the same reason, which is that they're
dealing with a raw pointer that must be valid to load/store memory to.
Note that the are no alignment requirements on any of these intrinsics.