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/wallet_esplora_async/src/main.rs

@@ -60,7 +60,7 @@ async fn main() -> Result<(), Box<dyn std::error::Error>> {
     let get_heights = wallet.tx_graph().missing_blocks(wallet.local_chain());
     let chain_update = client.update_local_chain(prev_tip, get_heights).await?;
     let update = LocalUpdate {
-        keychain: last_active_indices,
+        last_active_indices,
         graph: update_graph,
         ..LocalUpdate::new(chain_update)
     };