fix: improve more docs and more refactoring

Thank you @vladimirfomene for these suggestions.

* Rename `LocalUpdate::keychain` to `LocalUpdate::last_active_indices`.
* Change docs for `CheckPoint` to be more descriptive.
* Fix incorrect logic in `update_local_chain` for `EsploraExt` and
  `EsploraAsyncExt`.

example-crates/example_electrum/src/main.rs

@@ -278,7 +278,9 @@ fn main() -> anyhow::Result<()> {
 
         let indexed_additions = {
             let mut additions = IndexedAdditions::<ConfirmationHeightAnchor, _>::default();
-            let (_, index_additions) = graph.index.reveal_to_target_multi(&final_update.keychain);
+            let (_, index_additions) = graph
+                .index
+                .reveal_to_target_multi(&final_update.last_active_indices);
             additions.append(IndexedAdditions {
                 index_additions,
                 ..Default::default()