Clarify edge cases for Barrier::new

... since n-1 is undefined when the usize n is 0.

Author: tisonkun

library/std/src/sync/barrier.rs

@@ -68,6 +68,8 @@ impl Barrier {
     /// A barrier will block `n`-1 threads which call [`wait()`] and then wake
     /// up all threads at once when the `n`th thread calls [`wait()`].
     ///
+    /// If `n` is 0, the barrier acts the same as `n` is 1.
+    ///
     /// [`wait()`]: Barrier::wait
     ///
     /// # Examples