fix: Even more refactoring to code and documentation

Thank you to @LLFourn and @danielabrozzoni for these suggestions.

example-crates/wallet_esplora/src/main.rs

@@ -56,8 +56,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
 
     let (update_graph, last_active_indices) =
         client.update_tx_graph(keychain_spks, None, None, STOP_GAP, PARALLEL_REQUESTS)?;
-    let get_heights = wallet.tx_graph().missing_heights(wallet.local_chain());
-    let chain_update = client.update_local_chain(prev_tip, get_heights)?;
+    let missing_heights = wallet.tx_graph().missing_heights(wallet.local_chain());
+    let chain_update = client.update_local_chain(prev_tip, missing_heights)?;
     let update = LocalUpdate {
         last_active_indices,
         graph: update_graph,