Yes, that documentation is poorly worded.
What I think it means is that you cannot use:
NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] init];
[moc setParentContext:[self managedObjectContext]];
Which I have tested previously and it did fail with an error. YMMV of course as I tested it in iOS 5.x.
That is different than:
NSManagedObjectContext * localMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSConfinementConcurrencyType];
[localMOC setParentContext:[self mainContext]];
Which works just fine and is part of the newer Core Data design.
There is a subtle difference between the two initializers and the older, historic, initializer is missing some key component to allow parent-child contexts to work properly.