Add index entry "I/O control, buffering" for fdopen().

Doc/lib/libos.tex

@@ -228,6 +228,7 @@ These functions create new file objects.
 
 \begin{funcdesc}{fdopen}{fd\optional{, mode\optional{, bufsize}}}
 Return an open file object connected to the file descriptor \var{fd}.
+\index{I/O control!buffering}
 The \var{mode} and \var{bufsize} arguments have the same meaning as
 the corresponding arguments to the built-in \function{open()}
 function.