Add documentation to process::Child fields

[JOT85]

Motivation

I was using tokio::process::Child without much experience of std::process::Child and was reading the tokio docs. I worked out that I needed to use the child.stdin.take().unwrap() pattern but it would have saved me some time if it was in the docs. I later discovered that this was added to the rust std lib documentation by rust-lang/rust#74192.

Solution

Updated the doc comments on the public fields of Child to match the changes made by rust-lang/rust#74192.

[Darksonn]

Let's not have tests that fail to compile.

Suggested change

	/// ```compile_fail,E0425
	/// let stdin = child.stdin.take().unwrap();
	/// ```
	/// ```no_run
	/// # let child: tokio::process::Child = unreachable!();
	/// let stdin = child.stdin.take().unwrap();
	/// ```

[JOT85]

Using the code suggested errors because the second line in unreachable, what do prefer to do? Set it to ignore rather than no_run?

[Darksonn]

Just throw in some dummy command instead.

let child = tokio::process::Command::new("echo").spawn().unwrap();

[JOT85]

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