isdn: document Kernel CAPI driver interface

Create a file Documentation/isdn/INTERFACE.CAPI describing the
interface between the kernel CAPI subsystem and ISDN device drivers,
analogous to the existing Documentation/isdn/INTERFACE for the old
isdn4linux subsystem. Also add kerneldoc comments to the exported
functions in drivers/isdn/capi/kcapi.c.

Impact: Documentation
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>