clarify empty_nodes_mask documentation

miden-crypto/src/merkle/sparse_path.rs

@@ -25,6 +25,8 @@ use super::{
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct SparseMerklePath {
     /// A bitmask representing empty nodes. The set bit corresponds to the depth of an empty node.
+    /// The least significant bit (bit 0) describes depth 1 node (root's children).
+    /// The `bit index + 1` is equal to node's depth.
     empty_nodes_mask: u64,
     /// The non-empty nodes, stored in depth-order, but not contiguous across depth.
     nodes: Vec<RpoDigest>,