merge from 3.3

Clarify mmap.close method  behavior.  Addresses issue  #18815
Patch contributed by Anoop Thomas Mathew.
diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index 1f36cec..7db60d2 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -155,8 +155,9 @@
 
    .. method:: close()
 
-      Close the file.  Subsequent calls to other methods of the object will
-      result in an exception being raised.
+      Closes the mmap. Subsequent calls to other methods of the object will
+      result in a ValueError exception being raised. This will not close
+      the open file.
 
 
    .. attribute:: closed