Small Dispatchers.Main documentation inaccuracies with the new MM

[BoD]

On the doc for the Main dispatcher it says:

• "On JS and Native it is equivalent to the Default dispatcher.", which unless I'm mistaken is no longer true with the new MM (at least on Apple I can confirm that Default and Main are not the same)
• "MainCoroutineDispatcher.immediate is not supported on the Native platform" again it looks like it's no longer true with the new MM

Of course I understand that the doc may not want to be too "new MM specific" as it is still experimental. Maybe the differences can be noted explicitly.

[dkhalanskyjb]

Good point, we should change the docs, especially given that we're dropping support for the old memory model.