Improve docs for at_latest (#2035)

Tell the user that these functions operate with the last finalized block.

Co-authored-by: James Wilson <james@jsdw.me>

subxt/src/runtime_api/runtime_client.rs

@@ -40,7 +40,7 @@ where
         RuntimeApi::new(self.client.clone(), block_ref.into())
     }
 
-    /// Obtain a runtime API interface at the latest block hash.
+    /// Obtain a runtime API interface at the latest finalized block.
     pub fn at_latest(
         &self,
     ) -> impl Future<Output = Result<RuntimeApi<T, Client>, Error>> + Send + 'static {