Add a note to os.chown that permission constants can be combined

Doc/lib/libos.tex

@@ -753,7 +753,8 @@ Availability: Macintosh, \UNIX.
 \begin{funcdesc}{chmod}{path, mode}
 Change the mode of \var{path} to the numeric \var{mode}.
 \var{mode} may take one of the following values
-(as defined in the \module{stat} module):
+(as defined in the \module{stat} module) or bitwise or-ed
+combinations of them:
 \begin{itemize}
   \item \code{S_ISUID}
   \item \code{S_ISGID}