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.
> 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.

This wording seems ambiguous to me. The sentence proposes an assumed hypothetical situation (*"If one were to spawn node as a child process,"*), and then ascribes to node the wrapping of those descriptors into streams -- but which node is responsible for wrapping the streams? Does child process let you spawn anything, or just other node processes? For new programmers this ambiguity can really be a stumbling block. 

On the other hand, I might just be overthinking it -- feel free to tell me I'm being silly here :)