Add documentaton about atomicFu behaviour without plugin applying

[vdshb]

I've found no word about using atomicFu without gradle/compiler plugin in the documentation. I quickly looked at AtomicRef platform implementations and it seems they are thread-safe themself. I havn't anailsed the performance difference (but would like to hear about it from authors). I believe it would be benefitial to add a sentence or two about this case into the documentation.