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 58d00c8..ac1963f 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -152,8 +152,9 @@
 
    .. 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.
 
 
    .. method:: find(string[, start[, end]])