nrf51822/timer: Add some documentation and references
examples/timing: Modify example to test more APIs

This patch modifies the timeout function of the first timer to call
timer_stop() after 5 seconds and adds a third timer to test the
TIMER_SINGLESHOT type.
radio: Add documentation about radio events
nrf51822/radio: Update reference
nrf51822/radio: Document radio_set_tx_power()
nrf51822/radio: Document radio_init()
nrf51822/radio: Document ch2freq()
nrf51822/radio: Document radio_send()
nrf51822/radio: Document RADIO_IRQHandler()
examples/radio-observer: Fix length check

This patch fixes the minimum length check and documents it.
examples/radio-broadcaster: Fix type
examples/radio-broadcaster: Document symbols and PDU
examples/radio-broadcaster: Improve callback handling

This patch modifies the code to only one of the callbacks call
radio_send().
nrf51822/timer: Better error handling
ll: Add missing const modifier in ll_advertise_start()
ll: Check for errors in ll_advertise_start()
ll: Check for errors in ll_advertise_stop()
ll: Check for errors in ll_init()