Updated descriptions to incorporate additional information from Tim Peters.

This mostly closes SF bug #128251.

Doc/lib/libmmap.tex

@@ -20,25 +20,27 @@ different on Unix and on Windows.
 \begin{funcdesc}{mmap}{fileno, length\optional{, tagname}}
 \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.
 If you wish to map an existing Python file object, use its
 \method{fileno()} method to obtain the correct value for the
-\var{fileno} parameter.
-
-\var{tagname}, if specified, is a string giving a tag name for the mapping. 
-Windows allows you to have many different mappings against the same
-file.  If you specify the name of an existing tag, that tag is opened,
-otherwise a new tag of this name is created.  If this parameter is
-None, the mapping is created without a name.  Avoiding the use of the
-tag parameter will assist in keeping your code portable between Unix
-and Windows.
+\var{fileno} parameter.  The file must be opened for update.
+
+\var{tagname}, if specified and not \code{None}, is a string giving a
+tag name for the mapping.  Windows allows you to have many different
+mappings against the same file.  If you specify the name of an
+existing tag, that tag is opened, otherwise a new tag of this name is
+created.  If this parameter is omitted or \code{None}, the mapping is
+created without a name.  Avoiding the use of the tag parameter will
+assist in keeping your code portable between \UNIX{} and Windows.
 \end{funcdesc}
 
 \begin{funcdesc}{mmap}{fileno, size\optional{, flags, prot}}
-\strong{(Unix version)}  Maps \var{length} bytes from the file
+\strong{(\UNIX{} version)}  Maps \var{length} bytes from the file
 specified by the file handle \var{fileno}, and returns a mmap object.
 If you wish to map an existing Python file object, use its
 \method{fileno()} method to obtain the correct value for the
-\var{fileno} parameter.
+\var{fileno} parameter.  The file must be opened for update.
 
 \var{flags} specifies the nature of the mapping.  
 \constant{MAP_PRIVATE} creates a private copy-on-write mapping, so