Add comment for unix::Client::Fifo::is_non_blocking

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/unix.rs

@@ -24,6 +24,8 @@ pub enum Client {
     Fifo {
         file: File,
         path: PathBuf,
+        /// it can only go from false -> true but not the other way around, since that
+        ///  could cause a race condition.
         is_non_blocking: AtomicBool,
     },
 }