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.
Ah, interesting. I read the docs above as saying that you could pass raw fd integers into stdio. From the docs:

> 5\. Positive integer - The integer value is interpreted as a file descriptor that is is currently open in the parent process. It is shared with the child process, similar to how Stream objects can be shared.