Description about `Layout::array` differs from std

[Luv-Ray]

As Layout::array's document said, when the total size would exceed isize::MAX, returns LayoutError. Not the usize::MAX.

https://github.com/rust-lang/nomicon/blob/0d5f88475fe285affa6dbbc806e9e44d730797c0/src/vec/vec-alloc.md?plain=1#L178-L182