docs(): add in-code documentation regarding the solution taken

lib/bull.explorer.ts

@@ -32,7 +32,19 @@ export class BullExplorer implements OnModuleInit {
       .getProviders()
       .filter((wrapper: InstanceWrapper) =>
         this.metadataAccessor.isQueueComponent(
-          wrapper.inject || !wrapper.metatype
+          // NOTE: Regarding the ternary statement below,
+          // - The condition `!wrapper.metatype` is because when we use `useValue`
+          // the value of `wrapper.metatype` will be `null`.
+          // - The condition `wrapper.inject` is needed here because when we use
+          // `useFactory`, the value of `wrapper.metatype` will be the supplied
+          // factory function.
+          // For both cases, we should use `wrapper.instance.constructor` instead
+          // of `wrapper.metatype` to resolve processor's class properly.
+          // But since calling `wrapper.instance` could degrade overall performance
+          // we must defer it as much we can. But there's no other way to grab the
+          // right class that could be annotated with `@Processor()` decorator
+          // without using this property.
+          !wrapper.metatype || wrapper.inject
             ? wrapper.instance?.constructor
             : wrapper.metatype,
         ),
@@ -45,6 +57,8 @@ export class BullExplorer implements OnModuleInit {
         name: queueName,
       } =
         this.metadataAccessor.getQueueComponentMetadata(
+          // NOTE: We are relying on `instance.constructor` to properly support
+          // `useValue` and `useFactory` providers besides `useClass`.
           instance.constructor || metatype,
         );