docs: clarify CheckPoint and walk_descendants and SpkIterator behavior

crates/chain/src/spk_iter.rs

@@ -112,6 +112,8 @@ where
             return None;
         }
 
+        // `new_with_range` caps `end` at `BIP32_MAX_INDEX + 1`, and the check above
+        // ensures `next_index < end`, so the index is within the non hardened range.
         let script = self
             .descriptor
             .borrow()

crates/chain/src/tx_graph.rs

@@ -519,6 +519,8 @@ impl<A: Clone + Ord> TxGraph<A> {
 
     /// Creates an iterator that filters and maps descendants from the starting `txid`.
     ///
+    /// The starting transaction is excluded from the iterator.
+    ///
     /// The supplied closure takes in two inputs `(depth, descendant_txid)`:
     ///
     /// * `depth` is the distance between the starting `txid` and the `descendant_txid`. I.e., if

crates/core/src/checkpoint.rs

@@ -95,6 +95,8 @@ fn walk_to_floor<D>(current: &CheckPoint<D>, target_height: u32) -> Option<Check
 
 /// A checkpoint is a node of a reference-counted linked list of [`BlockId`]s.
 ///
+/// The list is guaranteed to contain at least one checkpoint.
+///
 /// Checkpoints are cheaply cloneable and are useful to find the agreement point between two sparse
 /// block chains.
 #[derive(Debug)]