Update documentation

* Add `warn(missing_docs)` for `bdk_wallet` and `bdk_chain`.
* Add missing documentation.
* Remove `LocalChain::heights` method.
* Remove old TODOs.

crates/electrum/src/electrum_ext.rs

@@ -236,7 +236,6 @@ impl ElectrumExt<ConfirmationHeightAnchor> for Client {
 
             populate_with_txids(self, anchor_block, &mut update, &mut txids.iter().cloned())?;
 
-            // [TODO] cache transactions to reduce bandwidth
             let _txs = populate_with_outpoints(
                 self,
                 anchor_block,