Ignore invalid reference arguments in collections

[xabbuh]

#6511 adds a description for the DependencyInjection component's feature to ignore missing arguments in method calls. symfony/symfony#17132 expanded this to also drop invalid references when being passed as part of a collection argument.

[wouterj]

Hmm, I'm wondering if it makes sense to document this as it seems very detailed and not really usefull to 99% of the readers.

[xabbuh]

I think we can simply add a short sentence to the recently added documentation.