document BB-758 potential bug on offsetsStore

Issue: BB-752

Author: SylvainSenechal

lib/BackbeatConsumer.js

@@ -823,6 +823,7 @@ class BackbeatConsumer extends EventEmitter {
         });
         // ensure consumer is active when calling offsetsStore() on
         // it, to avoid raising an exception (invalid state)
+        // TODO : potential issue here, see BB-758
         if (committableOffset !== null && !this.isPaused()) {
             this._consumer.offsetsStore([{ topic, partition,
                                            offset: committableOffset }]);