update TIOCGPTN comment

src/header/sys_ioctl/constants.rs

@@ -55,7 +55,8 @@ pub const TIOCSPTLCK: c_ulong = 0x4004_5431;
 /// location pointed to by `lock`.
 pub const TIOCGPTLCK: c_ulong = 0x8004_5439;
 
-// Get the name of the PT as a c_int.
+/// Obtain device unit number, which can be used to generate the filename of
+/// the pseudo-terminal slave device.
 pub const TIOCGPTN: c_ulong = 0x8004_5430;
 /// POSIX `sockatmark()` from `sys/socket.h` is intended to replace this.
 pub const SIOCATMARK: c_ulong = 0x8905;