net: context: Clarify net_context_send() API documentation

The net_context_send API documentation was missing the fact,
that the callback might be called after the net_context_send()
has returned. Also currently the timeout value is not used properly
anywhere in the code. This is left like this as there is not much
use for the timeout value atm. This might be fixed later if there
is proper use case for it.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>