[Container] add a note about _instanceof in the service tags doc

[alsciende]

_instanceof is often used to add a tag to all services implementing an interface, as documented at https://symfony.com/blog/new-in-symfony-3-3-simpler-service-configuration.

In consequence, it would be very useful to add a note about that option in the documentation about autoconfiguring service tags at https://symfony.com/doc/current/service_container/tags.html#autoconfiguring-tags.

[javiereguiluz]

Fixed by #10491.