Minor clarification, saying that blocking means no timeout (from bug #882297)

Doc/lib/libsocket.tex

@@ -548,7 +548,7 @@ are described in \ref{bltin-file-objects}, ``File Objects.'')
 The file object references a \cfunction{dup()}ped version of the
 socket file descriptor, so the file object and socket object may be
 closed or garbage-collected independently.
-The socket must be in blocking mode.
+The socket must be in blocking mode (it can not have a timeout).
 \index{I/O control!buffering}The optional \var{mode}
 and \var{bufsize} arguments are interpreted the same way as by the
 built-in \function{file()} function; see ``Built-in Functions''