SF 964876 mapping a 0 length file

Document that it can't be done on Windows.

Doc/lib/libmmap.tex

@@ -42,9 +42,11 @@ the underlying file.
 \begin{funcdesc}{mmap}{fileno, length\optional{, tagname\optional{, access}}}
   \strong{(Windows version)} Maps \var{length} bytes from the file
   specified by the file handle \var{fileno}, and returns a mmap
-  object.  If \var{length} is \code{0}, the maximum length of the map
-  will be the current size of the file when \function{mmap()} is
-  called.
+  object.  If \var{length} is larger than the current size of the file,
+  the file is extended to contain \var{length} bytes.  If \var{length}
+  is \code{0}, the maximum length of the map is the current size
+  of the file, except that if the file is empty Windows raises an
+  exception (you cannot create an empty mapping on Windows).
 
   \var{tagname}, if specified and not \code{None}, is a string giving
   a tag name for the mapping.  Windows allows you to have many