[Config] Clarify the difference between lists and maps

[xabbuh]

It seems that it is a bit unclear when the Config component processes an array node in the tree builder either as a map or as a list. Especially the effects of useAttributeAsKey() should be clarified (see symfony/symfony#12304 for the context).

[wouterj]

This should be done in components/config/definition

[vishalraj82]

I am using useAttributeAsKey, but could not find any way for the child keys to have only boolean values. Any help ?

[xabbuh]

@vishalraj82 We only use GitHub to keep track of what needs to be done in the documentation. I suggest you use one of the support channels to reach out for help. Thank you for understanding.