Service Subscribers & Locators/Indexing the Collection of Services: Code example should include the "index_by" key

[codegain]

In symfony/symfony#34637 I pointed out that default_index_method without index_by is useless and the indexing logic will not be triggered.

The code example below If you prefer to use another method name, add a default_index_method attribute to the locator service defining the name of this custom method: should include the index_by key with some value other than null.

[HeahDude]

Closing as fixed by #13099. Thanks