Add documentation for openpty

src/pty.rs

@@ -13,6 +13,13 @@ pub struct OpenptyResult {
     pub slave: RawFd,
 }
 
+/// Create a new pseudoterminal, returning the slave and master file descriptors
+/// in `OpenptyResult`
+/// (see [openpty](http://man7.org/linux/man-pages/man3/openpty.3.html)). 
+///
+/// If `winsize` is not `None`, the window size of the slave will be set to
+/// the values in `winsize`. If `termios` is not `None`, the pseudoterminal's
+/// terminal settings of the slave will be set to the values in `termios`.
 #[inline]
 pub fn openpty(winsize: Option<Winsize>, termios: Option<Termios>) -> Result<OpenptyResult> {
     let mut slave: libc::c_int = -1;