update Documentation/isdn/00-INDEX

After the merging of mISDN, state which files refer only to the
old isdn4linux subsystem.  Also add a few missing files.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>