add a mention of OctreeChunkIndex inside of the ChunkMap docs
//! A sparse lattice map made of up array chunks.
//!
-//! # Addressing
+//! # Access
//!
//! The data can either be addressed by chunk key with the `get_chunk*` methods or by individual points using the `Get*` and
//! `ForEach*` trait impls. The map of chunks uses `Point3i` keys. The key for a chunk is the minimum point in that chunk, which
//! is always a multiple of the chunk shape. Chunk shape dimensions must be powers of 2, which allows for efficiently
//! calculating a chunk key from any point in the chunk.
//!
+//! If you require iteration over large, but very sparse regions, you might be better off using an `OctreeChunkIndex` to track
+//! the set of occupied chunks. Traversing that index can be faster than doing hash map lookups on all of the possible chunks
+//! in a region.
+//!
//! # Chunk Storage
//!
//! `ChunkMap<N, T, Meta, Store>` depends on a backing chunk storage `Store`, which can implement some of `ChunkReadStorage` or
