Document `high_level::load_cell_data` failure mode

[phroi]

As @XuJiandong noticed:

The load_cell_data function can load all cell data at once. However, the cell data might be very large, potentially consuming all available memory in certain scenarios. It is recommended to use the following low-level syscall to load only a portion of the cell data

Could the load_cell_data docs be updated to make evident this failure mode?

Keep up the great work :muscle: Phroi