select_nth documentation is insufficient

[TheIronBorn]

The documentation's description of the return values is slightly incorrect.

rust/library/core/src/slice/mod.rs

Lines 2631 to 2633 in c845946

	/// element" in other libraries. It returns a triplet of the following values: all elements less
	/// than the one at the given index, the value at the given index, and all elements greater than
	/// the one at the given index.

The following code however, produces ([1, 1], 1, [1, 1]):

let mut v = [1i32; 5];
let (x, y, z) = v.select_nth_unstable(2);

(playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3c68d77916745c6442866324d675755d)

[kartva]

@rustbot claim

[kartva]

Oh, it seems there's already an active PR for this. Sorry!