Add details about the return value for mmap.flush().
diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index ca0a2eb..d2a6d1e 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -162,6 +162,12 @@
changes to the given range of bytes will be flushed to disk; otherwise, the
whole extent of the mapping is flushed.
+ **(Windows version)** A nonzero value returned indicates success; zero
+ indicates failure.
+
+ **(Unix version)** A zero value is returned to indicate success. An
+ exception is raised when the call failed.
+
.. method:: mmap.move(dest, src, count)