When stating that some parameters to makefile() are similar to the open()

parameters, given a hyperlink to the right part of the documentation to
make it easier to look those up.  Also, refer to the file() function/
constructor instead of open() now that that is where the actual docs for
those parameters live.
This closes SF bug #472004.

Doc/lib/libsocket.tex

@@ -452,7 +452,8 @@ socket file descriptor, so the file object and socket object may be
 closed or garbage-collected independently.
 \index{I/O control!buffering}The optional \var{mode}
 and \var{bufsize} arguments are interpreted the same way as by the
-built-in \function{open()} function.
+built-in \function{file()} function; see ``Built-in Functions''
+(section \ref{built-in-funcs}) for more information.
 \end{methoddesc}
 
 \begin{methoddesc}[socket]{recv}{bufsize\optional{, flags}}