Fix child process docs

Fix a number of outright errors in child process documentation, as well as improve the general readability of it by using hyperlinks, documenting defaults, ordering the docs consistently, etc.
That's fair. The reason I was looking for that reorganization is that the new paragraph pushes down the example snippet, while leading into it with a sentence describing a feature that is not often utilized.

> Descriptors greater than 2 can have a stream associated with them by
> constructing a ['net.Socket'](net.html#net_new_net_socket_options) with the `fd`
> option, be passed directly to [fs](fs.html) APIs if they are files, etc.
>
> Example:

It definitely struck me as something that needs a code example, and if I were coming to the docs fresh, I could definitely see myself losing a few moments reconciling that sentence with the succeeding example code. However, you're right, it doesn't make sense to immediately launch a paragraph about non-stdio fd's with a sentence describing fds.

If it's the case that sharing non-stdio fds are not widely supported, then we can create the aforementioned level-4 subsection describing them and how they're not available on windows.