Rollup merge of #44536 - Havvy:transmute-docs, r=steveklabnik

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

This fixes the second bullet point on #44281 and also removes some incorrect information.

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.
     ///

src/libcore/intrinsics.rs

@@ -848,12 +848,12 @@ extern "rust-intrinsic" {
     /// // The no-copy, unsafe way, still using transmute, but not UB.
     /// // This is equivalent to the original, but safer, and reuses the
     /// // same Vec internals. Therefore the new inner type must have the
-    /// // exact same size, and the same or lesser alignment, as the old
+    /// // exact same size, and the same alignment, as the old type.
-    /// // type. The same caveats exist for this method as transmute, for
+    /// // The same caveats exist for this method as transmute, for
     /// // the original inner type (`&i32`) to the converted inner type
     /// // (`Option<&i32>`), so read the nomicon pages linked above.
     /// let v_from_raw = unsafe {
-    ///     Vec::from_raw_parts(v_orig.as_mut_ptr(),
+    ///     Vec::from_raw_parts(v_orig.as_mut_ptr() as *mut Option<&i32>,
     ///                         v_orig.len(),
     ///                         v_orig.capacity())
     /// };