doc: clarify child_process error behaviour

Hi,

I was wondering whether or not the `exit` event would still be triggered after an `error` event on a child-process. Since I couldn't find this information in the documentation I wrote some tests in the node console and arrived in the conclusion that no more events will be triggered at all.

I still would like this to be in the documentation because 1) it's easier to find there and 2) since that behaviour otherwise would be undocumented I think that it would be bad practice relying on it.

The use case I had, which I think should be fairly common, is having a function that takes a callback and spawns a child process. E.g.

```javascript
function cp(src, dest, cb) {
  var child = spawn('cp', [src, dest], { stdio: 'ignore' });
  child.on('exit', function (code) {
    if (code === 0) {
      cb(null);
    } else {
      cb(new Error('Unknown exit code'));
    }
  });
}
```

Now, the problem with this code is that it will throw an unhandled exception if `cp` can't be found in the path. Naturally  one would fix this with the following line:

```javascript
  child.on('error', cb);
```

Everything is good, except that if the behaviour would change in the future (remember, it isn't specified anywhere) and the `exit` event would still be triggered after the `error` event, the callback would be called twice. Not good.

I hope that I managed to explain the problem and please comment with any questions, I will try and respond asap. Thanks for your time!