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.
This doesn't make sense, you'd have:

```
#### Sharing non-stdio file descriptors

In the child, file descriptors 0 - 2 are automatically wrapped into streams by node, and made
available as `process.stdin`, `process.stdout`, and `process.stderr`,
respectively. ...
```

That's a heading saying "non-studio", followed by a sentence documenting ***stdio***.