fix: docs and some minor refactoring

Shout out to @LLFourn for these suggestions.

* Improve/fix `LocalChain` documentation
* Refactor `TxGraph::missing_blocks` to make it more explicit that
  `last_block` has state.
* `update_local_chain` method of `EsploraExt` and `EsploraAsyncExt` now
  returns a `local_chain::Update` instead of just a `CheckPoint`.

crates/esplora/src/blocking_ext.rs

@@ -6,8 +6,8 @@ use bdk_chain::collections::{BTreeMap, BTreeSet};
 use bdk_chain::{
     bitcoin::{BlockHash, Script},
     local_chain::{self, CheckPoint},
+    BlockId, ConfirmationTimeAnchor, TxGraph,
 };
-use bdk_chain::{BlockId, ConfirmationTimeAnchor, TxGraph};
 use esplora_client::{Error, TxStatus};
 
 use crate::{anchor_from_status, ASSUME_FINAL_DEPTH};