add comment explaining that failed media keys are logged

src/service/admin.rs

@@ -809,6 +809,7 @@ impl Service {
                 let mut failed_keys = 0;
                 while let Some(mxc) = rx.recv().await {
                     let Ok(mxc) = mxc else {
+                        // Error details are logged by media::iter_all
                         failed_keys += 1;
                         continue;
                     };