message: mmapheap: document OneWayHeap
diff: namespace mesh {
  +// OneWayMmapHeap allocates address space through calls to mmap and
  +// will never unmap address space.
  class OneWayMmapHeap {
  private:
  DISALLOW_COPY_AND_ASSIGN(OneWayMmapHeap);
  @@ -89,6 +91,8 @@ public:
  }
  };
  +// MmapHeap extends OneWayMmapHeap to track allocated address space
  +// and will free memory with calls to munmap.
  class MmapHeap : public OneWayMmapHeap {
  private:
  DISALLOW_COPY_AND_ASSIGN(MmapHeap);
  