Rollup merge of rust-lang#95016 - janpaul123:patch-1, r=dtolnay

Docs: make Vec::from_raw_parts documentation less strict

This is my first PR; be gentle!

In https://users.rust-lang.org/t/why-does-vec-from-raw-parts-require-same-size-and-not-same-size-capacity/73036/2?u=janpaul123 it was suggested to me that I should make a PR to make the documentation of `Vec::from_raw_parts` less strict, since we don't require `T` to have the same size, just `size_of::<T>() * capacity` to be the same, since that is what results in `Layout::size` being the same in `dealloc`, which is really what matters.

Also in https://users.rust-lang.org/t/why-does-vec-from-raw-parts-require-same-size-and-not-same-size-capacity/73036/8?u=janpaul123 it was suggested that it's better to use `slice::from_raw_parts`, which I think is useful advise that could also be mentioned in the docs, so I added that too.

Let me know what you think! :)

library/alloc/src/vec/mod.rs

@@ -479,20 +479,24 @@ 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).
- /// * `T` needs to have the same size and alignment as what `ptr` was allocated with.
+ /// * `T` needs to have the same alignment as what `ptr` was allocated with.
  /// (`T` having a less strict alignment is not sufficient, the alignment really
  /// needs to be equal to satisfy the [`dealloc`] requirement that memory must be
  /// allocated and deallocated with the same layout.)
+ /// * The size of `T` times the `capacity` (ie. the allocated size in bytes) needs
+ /// to be the same size as the pointer was allocated with. (Because similar to
+ /// alignment, [`dealloc`] must be called with the same layout `size`.)
  /// * `length` needs to be less than or equal to `capacity`.
- /// * `capacity` needs to be the capacity that the pointer was allocated with.
  ///
  /// Violating these may cause problems like corrupting the allocator's
  /// internal data structures. For example it is **not** safe
  /// to build a `Vec<u8>` from a pointer to a C `char` array with length `size_t`.
  /// It's also not safe to build one from a `Vec<u16>` and its length, because
  /// the allocator cares about the alignment, and these two types have different
  /// alignments. The buffer was allocated with alignment 2 (for `u16`), but after
- /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1.
+ /// turning it into a `Vec<u8>` it'll be deallocated with alignment 1. To avoid
+ /// these issues, it is often preferable to do casting/transmuting using
+ /// [`slice::from_raw_parts`] instead.
  ///
  /// The ownership of `ptr` is effectively transferred to the
  /// `Vec<T>` which may then deallocate, reallocate or change the