Document the new additional signature for utime(). In addition to the

previous functionality utime(path, (atime, mtime)), now allowed is
utime(path, None) which sets the file's times to the current time.

Doc/lib/libos.tex

@@ -702,9 +702,12 @@ Remove the file \var{path}.  This is the same function as
 Availability: Macintosh, \UNIX{}, Windows.
 \end{funcdesc}
 
-\begin{funcdesc}{utime}{path, (atime, mtime)}
-Set the access and modified time of the file to the given values.
-(The second argument is a tuple of two items.)
+\begin{funcdesc}{utime}{path, times}
+Set the access and modified times of the file specified by \var{path}.
+If \var{times} is \code{None}, then the file's access and modified
+times are set to the current time.  Otherwise, \var{times} must be a
+2-tuple of numbers, of the form \var{(atime, mtime)} which is used to
+set the access and modified times, respectively.
 Availability: Macintosh, \UNIX{}, Windows.
 \end{funcdesc}