Update unix.rs

Co-authored-by: Weihang Lo <weihanglo@users.noreply.github.com>

src/unix.rs

@@ -17,6 +17,10 @@ use std::thread::{self, Builder, JoinHandle};
 use std::time::Duration;
 
 #[derive(Debug)]
+/// This preserves the `--jobserver-auth` type at creation time,
+/// so auth type will be passed down to and inherit from sub-Make processes correctly.
+///
+/// See <https://github.com/rust-lang/jobserver-rs/issues/99> for details.
 enum ClientCreationArg {
     Fds { read: c_int, write: c_int },
     Fifo(Box<Path>),