Explain OUI DevAddr Ranges Better

[lthiery]

[mikev]

@samgutentag - This ticket is still relevant.

I think the docs explanation is supposed to be here - https://docs.helium.com/lorawan-on-helium/

But this is technically not correct: It is even possible to multiplex many devices on the same DevAddr at the same time, using the Message Integrity Check (MIC) to disambiguate.