Documentation: expand isdn/INTERFACE.CAPI document

- Note that send_message() may be called in interrupt context.
- Describe the storage of CAPI messages and payload data in SKBs.
- Add more details to the description of the _cmsg structure.
- Describe kernelcapi debugging output.

Impact: documentation
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>