caif: Update documentation of CAIF transmit and receive functions.

Trivial patch updating documentation in header files only.
Error handling of CAIF transmit errors was changed by commit:
      caif: Don't resend if dev_queue_xmit fails.
This patch updates the documentation accordingly.

Signed-off-by: Sjur Brændeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>