Officially, the rule is simple. If the API is publicly available to free and non paid Apple developers, it can be asked about without any issue. This actually covers around 90% of the developer documentation Apple provides.
For the other 10%, If the API is explicitly listed by Apple as being under NDA and/or in Beta, as was the case with, for example, iOS 5, start treading lightly. No one can prevent you from asking those questions, however, do not be surprised if Apple disables your paid account for violating the NDA*.
Also, your target audience is really limited to other developers under the same NDA, and some might not even be bothered to answer your question in public, and often the best answers for these questions are actually found at the Apple Developer Support forum.
* I personally know of 3 actual instances where this has happened, none from SO that I am aware of however.