add comment about padding chunks to surface_nets doc comment
//! let local_cache = LocalChunkCache::new();
//! let reader = ChunkMapReader3::new(&map, &local_cache);
//! for chunk_key in chunk_keys_to_update.into_iter() {
+//! // It's crucial that we pad the chunk so we have access to adjacent points during meshing.
//! let padded_chunk_extent = map.extent_for_chunk_at_key(&chunk_key).padded(1);
//! let mut padded_chunk = Array3::fill(padded_chunk_extent, 0.0);
//! copy_extent(&padded_chunk_extent, &reader, &mut padded_chunk);
+//!
//! let mut sn_buffer = SurfaceNetsBuffer::new(padded_chunk_extent.num_points());
//! surface_nets(&padded_chunk, &padded_chunk_extent, &mut sn_buffer);
//! // Do something with the mesh output...
