Document that CalledProcessError.returncode is the negative
signal number when the process died due to a signal.
diff --git a/Doc/library/subprocess.rst b/Doc/library/subprocess.rst
index 2ec94bf..67bee40 100644
--- a/Doc/library/subprocess.rst
+++ b/Doc/library/subprocess.rst
@@ -188,7 +188,8 @@
 
     .. attribute:: returncode
 
-        Exit status of the child process.
+        Exit status of the child process.  If the process exited due to a
+        signal, this will be the negative signal number.
 
     .. attribute:: cmd