Fix example in transmute; add safety requirement to Vec::from_raw_parts

src/liballoc/vec.rs

@@ -370,6 +370,7 @@ impl<T> Vec<T> {
     ///
     /// * `ptr` needs to have been previously allocated via [`String`]/`Vec<T>`
     ///   (at least, it's highly likely to be incorrect if it wasn't).
+    /// * `ptr`'s `T` needs to have the same size and alignment as it was allocated with.
     /// * `length` needs to be less than or equal to `capacity`.
     /// * `capacity` needs to be the capacity that the pointer was allocated with.
     ///