Improve documentation about decorating services

[kpodemski]

• add a notice that you can ignore decorating non-existing services, which should be helpful if you work on a module that has to work on many core versions, details below
• provide an example module

Fail silently:
decoration_on_invalid: ignore

details:
https://symfony.com/doc/4.4/service_container/service_decoration.html#control-the-behavior-when-the-decorated-service-does-not-exist