doc: Update

Author: DanSnow

src/sys/signal.rs

@@ -683,7 +683,7 @@ pub fn kill<T: Into<Option<Signal>>>(pid: ::unistd::Pid, signal: T) -> Result<()
 /// Send a signal to a process group [(see
 /// killpg(3))](http://pubs.opengroup.org/onlinepubs/9699919799/functions/killpg.html).
 ///
-/// If `pgrp` less then or equal 1, the behavior is undefined.
+/// If `pgrp` less then or equal 1, the behavior is platform-specific.
 /// If `signal` is `None`, `killpg` will only preform error checking and won't
 /// send any signal.
 pub fn killpg<T: Into<Option<Signal>>>(pgrp: ::unistd::Pid, signal: T) -> Result<()> {