doc: Update doc

Author: DanSnow

src/sys/signal.rs

@@ -682,8 +682,12 @@ 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).
-pub fn killpg<T: Into<Option<Signal>>>(pid: ::unistd::Pid, signal: T) -> Result<()> {
-    let res = unsafe { libc::killpg(pid.into(),
+///
+/// If `pgrp` less then or equal 1, the behavior is undefined.
+/// 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<()> {
+    let res = unsafe { libc::killpg(pgrp.into(),
                                   match signal.into() {
                                       Some(s) => s as libc::c_int,
                                       None => 0,

test/sys/test_signal.rs

@@ -11,7 +11,8 @@ fn test_kill_none() {
 
 #[test]
 fn test_killpg_none() {
-    killpg(getpgrp(), None).expect("Should be able to send signal to my process group.");
+    killpg(getpgrp(), None)
+        .expect("Should be able to send signal to my process group.");
 }
 
 #[test]