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`.

crates/bdk/src/wallet/mod.rs

@@ -1711,7 +1711,7 @@ impl<D> Wallet<D> {
         let (_, index_additions) = self
             .indexed_graph
             .index
-            .reveal_to_target_multi(&update.keychain);
+            .reveal_to_target_multi(&update.last_active_indices);
         changeset.append(ChangeSet::from(IndexedAdditions::from(index_additions)));
         changeset.append(ChangeSet::from(
             self.indexed_graph.apply_update(update.graph),