feat(docs): add multiton docs and fix the custom handler service

Signed-off-by: Juan Pablo Garcia Ripa <sarabadu@gmail.com>

docs/backend-system/architecture/03-services.md

@@ -223,6 +223,35 @@ export const customFooServiceFactory = createServiceFactory({
 
 This allows you to provide more advanced options for the service implementation that couldn't be expressed through static configuration. It also gives users of the service implementation access to other services through dependency injection, which can be useful for their customizations.
 
+## Multiton
+
+By default the service reference will point to a singleton instance of the service. This meand if a new service factory uses this reference will override the previous one. This is the most common use-case, but in some cases you may want to have multiple instances of the same service.
+For some services, is desirable to extend the functionality instead of overriding it. For example, some services could have many handler to address a specific event, and you may want to add a new handler instead of overriding the previous one. In this case, you can use the `multiton` option when creating the service reference:
+
+```ts
+// example-service-ref.ts
+import { createServiceRef } from '@backstage/backend-plugin-api';
+
+export interface FooService {
+  foo(options: FooOptions): Promise<FooResult>;
+}
+
+export const fooServiceRef = createServiceRef<FooService>({
+  id: 'example.foo',
+  multiton: true, // this service ref will be an array of instances
+});
+```
+
+When adding this serviceRef ad dependency to a factory, the factory will receive an array of instances instead of a single instance:
+
+```ts
+deps: {fooServices: fooServiceRef},
+  factory(fooServices) {
+    // fooServices is an array of instances
+    return new Bar(fooServices);
+  },
+```
+
 ## Service Factory Options Pattern
 
 :::note Note