registerRelevantVoiceInstructionsCallback
@MapboxExperimental
Registers a RelevantVoiceInstructionsCallback to receive the relevant voice instructions.
Note: the observer will be automatically unregistered after receiving the callback, as this is a one-time fetch operation. And if you need to get the relevant voice instructions again, simply call this method again.
Parameters
callback
the observer to register (will be auto-unregistered after callback)