Clarify os.strerror()s exception behavior.

diff --git a/Doc/library/os.rst b/Doc/library/os.rst
index 0790f644d87a80729adda154e0abf418644ebb1c..b2d8f8085732de55a63d0c794b6141a29fd77810 100644 (file)
--- a/Doc/library/os.rst
+++ b/Doc/library/os.rst
@@ -285,7 +285,8 @@ process and user.
 .. function:: strerror(code)
 
    Return the error message corresponding to the error code in *code*.
-   Availability: Unix, Windows.
+   On platforms where :cfunc:`strerror` returns ``NULL`` when given an unknown
+   error number, :exc:`ValueError` is raised.  Availability: Unix, Windows.
 
 
 .. function:: umask(mask)