Builder::worker_threads doesn't panic when used with a current_thread runtime

[hds]

Version

tokio v1.19.2 (also master branch)

Platform

Linux sync-z2 4.15.0-162-generic #170-Ubuntu SMP Mon Oct 18 11:38:05 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Description

The documentation for the function Builder::worker_threads states that if it is used
with current_thread runtime it will panic. However it doesn't panic, it appears to
ignore the value set by the call instead.

Documentation:
https://docs.rs/tokio/latest/tokio/runtime/struct.Builder.html#method.worker_threads

This test demonstrates the behaviour (it would live in tokio/tests):

#![warn(rust_2018_idioms)]
#![cfg(feature = "full")]

use tokio::runtime::Builder;

#[test]
#[should_panic]
fn test_worker_threads_panics() {
    let rt = Builder::new_current_thread()
        .worker_threads(4)
        .build()
        .unwrap();

    rt.block_on(async {
        tokio::spawn(async {}).await.unwrap();
    });
}

The test fails as nothing panics.

Additionally, the documentation for worker_threads contains 2 panic sections.
The other panic section describes a case that does panic (val = 0).

The 2 options I see here are:

1. Leave the functionality as it is (silently ignore worker_threads for a current_thread runtime) and fix the documentation
2. Change the functionality so that it does panic (ideally directly within worker_threads itself) and also fix the documentation so that all panics are described in a single heading

I would lean towards option 1 as this makes it easier to choose the runtime Kind in the builder at execution time. It will also not break any existing code.