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/events/events_client.rs

@@ -48,7 +48,7 @@ where
         self.at_or_latest(Some(block_ref.into()))
     }
 
-    /// Obtain events for the latest block.
+    /// Obtain events for the latest finalized block.
     pub fn at_latest(&self) -> impl Future<Output = Result<Events<T>, Error>> + Send + 'static {
         self.at_or_latest(None)
     }