Clarified new fsync() docs.

Doc/lib/libos.tex

@@ -448,14 +448,15 @@ Availability: \UNIX.
 \end{funcdesc}
 
 \begin{funcdesc}{fsync}{fd}
-Force write of file with filedescriptor \var{fd} to disk.
+Force write of file with filedescriptor \var{fd} to disk.  On \UNIX,
+this calls the native \cfunction{fsync()} function; on Windows, the
+MS \cfunction{_commit()} function.
 
-On Windows this calls the MS \cfunction{_commit()} function.  If you're
-starting with a Python file object \var{f}, first do
+If you're starting with a Python file object \var{f}, first do
 \code{\var{f}.flush()}, and then do \code{os.fsync(\var{f}.fileno()},
 to ensure that all internal buffers associated with \var{f} are written
 to disk.
-Availability: \UNIX, and Windows starting in 2.3.
+Availability: \UNIX, and Windows starting in 2.2.3.
 \end{funcdesc}
 
 \begin{funcdesc}{ftruncate}{fd, length}