Improve the documentation for #[tokio::test]

[gpyrros]

Improved documentation for #[tokio::test] to mirror the #[tokio::main] one

Motivation

Fixing issue #4720

Solution

Mirrored the #[tokio::main] documentation + explained that each test gets a separate current-thread runtime.

[Darksonn]

This is the third PR you've opened (#4760, #4759). Next time you change anything, please push another commit to the same PR instead of opening a new PR.

[gpyrros]

This is the third PR you've opened (#4760, #4759). Next time you change anything, please push another commit to the same PR instead of opening a new PR.

You are correct and I am very sorry. I just had to delete the first fork.

[Darksonn]

Suggested change

	///Equivalent code not using `#[tokio::test]`
	/// Equivalent code not using `#[tokio::test]`

[Darksonn]

Thanks!