Update comment

Author: giampaolo

Lib/subprocess.py

@@ -2125,13 +2125,13 @@ def _wait(self, timeout):
                 # Try efficient wait first.
                 if self._wait_pidfd(timeout) or self._wait_kqueue(timeout):
                     # Process is gone. At this point os.waitpid(pid, 0)
-                    # should return immediately, but in rare races the
-                    # PID may have been reused.
+                    # will return immediately, but in very rare races
+                    # the PID may have been reused.
                     # os.waitpid(pid, WNOHANG) ensures we attempt a
                     # non-blocking reap without blocking indefinitely.
                     with self._waitpid_lock:
                         if self.returncode is not None:
-                            return self.returncode
+                            return self.returncode  # Another thread waited.
                         (pid, sts) = self._try_wait(os.WNOHANG)
                         assert pid == self.pid or pid == 0
                         if pid == self.pid: