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.

nomicon/src/vec/vec-alloc.md

Lines 178 to 182 in 0d5f884

	// `Layout::array` checks that the number of bytes is <= usize::MAX,
	// but this is redundant since old_layout.size() <= isize::MAX,
	// so the `unwrap` should never fail.
	let new_layout = Layout::array::<T>(new_cap).unwrap();
	(new_cap, new_layout)