Obsolete comment about MemRegion allocation

The comment describing MemRegion talks about heap allocation in a few cases, with operator new and operator new[] provided by the class for those special cases. Those operators were removed by JDK-8238999, with the array case replaced by static member functions create_array and destroy_array. (There were no uses of individual heap allocated MemRegion objects.) The class description was not updated, and should be.