--killSignal description enhanced

Added information about the fact that no output is appended to the logs after forever stop/stopall is called.

Consider this:
```
process.on('SIGINT', function() {
   console.log("SIGINT invoked");
});
```
When the script is started with `forever start script.js --killSignal=SIGINT` the function will be invoked but "SIGINT invoked" will not appear in the log file.

This might result in endless debugging which I hope to prevent with the enhanced description about the `--killSignal` option.