add todo comment

Author: dconeybe

firebase-firestore/src/main/java/com/google/firebase/firestore/local/SQLiteRemoteDocumentCache.java

@@ -177,6 +177,7 @@ public Map<DocumentKey, MutableDocument> getAll(Iterable<DocumentKey> documentKe
     }
     backgroundQueue.drain();
 
+    // TODO(dconeybe): schedule the backfill asynchronously.
     documentTypeBackfills.backfill(db);
 
     // Synchronize on `results` to avoid a data race with the background queue.
@@ -277,6 +278,7 @@ private Map<DocumentKey, MutableDocument> getAll(
             });
     backgroundQueue.drain();
 
+    // TODO(dconeybe): schedule the backfill asynchronously.
     documentTypeBackfills.backfill(db);
 
     // Synchronize on `results` to avoid a data race with the background queue.