Add docs for array values in route defaults

[javiereguiluz]

In symfony/symfony#11394 @xabbuh added support for array values in route defaults. Should we document this?

[xabbuh]

I am not sure if we really need to document it. However, to give some context: The missing support for arrays for default values in the XML format was noticed when dealt with #4017.

[javiereguiluz]

@xabbuh OK then. Let's close this as "won't fix". Thanks.