API Docs: process

[steveklabnik]

Part of #29329

http://doc.rust-lang.org/std/process/

Here's what needs to be done to close out this issue:

• the module docs are okay but could use elaboration.
• abort needs an example, preferably showing off its destuctor behavior. (some elaboration here, and a first PR here)
• abort should also talk about its relationship with panic!, specifically, unwinding vs aborting panics.
• abort has a bug: The [abort] function terminates the process, the []s need to be dropped.
• Child the Note shouldn't be one, and should come before the examples.
• ChildStderr should have its second sentence as not a summary, needs some newlines.
• ChildStdin has the same issue.
• ChildStdout has the same issue.
• Command is good but many of its methods could use more elaboration in their docs. See below for some details.
• ExitStatus should link to status, and its code method needs an example.
• Output should link to output.
• Stdio should link to stdio and needs expanded explanations and examples.
• exit needs some examples to show its behavior. An example of how people use this in main would be fantastic too.

With regards to Command, @frewsxcv brought up in #40983 that

• stdin, stdout, and stderr should specify what the default behavior is if they aren't explicitly called.

And @retep998 mentions

Note that the default behavior depends on whether the child process is using the console subsystem or windows subsystem. Console subsystem apps will inherit your stdout/stdin/stderr while windows subsystem apps will start with no stdout/stdin/stderr unless you explicitly assign them.

[LukasKalbertodt]

This API really could use a better documentation. To list all failure cases of several functions would be great :)