[DI] Explain that "class FQCN as service IDs" doesn't always work

[javiereguiluz]

As explained in symfony/symfony#22851, you can't always use FQCN as service IDs. Specifically, classes that don't use namespaces won't work (e.g. anything related to Twig).

Although the error message is clear, let's make sure that we mention this limitation somewhere.

[stof]

Technically, you can still use the FQCN as service id (the id is just a string, and FQCNs are strings). But you will still need to configure the class explicitly for the service. What won't work for classes in the global namespace is the auto-configuration of the class based on the id.

[weaverryan]

👍 I didn't realize this limitation - but it makes sense!