Bluetooth: Enforce cooperative priorities in Bluetooth threads

The Bluetooth subsystem assumes execution of its system threads in
cooperative priority, including the system workqueue and the thread
that interact with the controller (i.e. calling bt_send). This commit
adds a compile-time check for the system workqueue priority and
documentation for the bt_send API call.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>