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.
Yes, you can do that. But if you do

```
stdio: [0,1,2, 'pipe']
```

In the child you have fd 3, a pipe... but you have no obvious way to use fd 3 as a stream.

In the parent, you have `child.stdio[3]`, which is a fd of a pipe... but again, same problem, no obvious way to use it.

And... I notice ChildProcess.stdio is an undocumented property :-(, I'll add it. And I think this note about wrapping in net.Socket should be on the docs of that property.