doc: several properties undocumented on child process

[martinheidegger]

Tracking back as far as Node 0.12 it seems that the documentation for ChildProcess is missing several properties.

• exitCode - if the process exited will contain the exit code, else null. Useful in combination with killed → if (proc.exitCode !== null || proc.killed) to see if the process is still running.
• signalCode - not sure what that is good for.
• spawnfile & spawnargs the initial properties of the spawning. In combination with exec will show what shell is used. Useful for debugging.

[sam-github]

signalCode - not sure what that is good for.

Its probably the pair of exitCode -- if process terminated, it was either because it exited or it was signalled, in which case one or the other would be set. So, your example above would probably only work if the child was signalled from the node parent. If it was signalled from elsewhere (the shell?), or was signalled by the OS (segfault, SIGHUP), proc.killed would probably not be set.