Add information about default included associations policy per adapter

[beauby]

The docs should be clear which associations will get serialized by default (Json/Attributes: only first-level associations, a.k.a '*', JsonApi: no associated resource, but all first-level relationship objects).

[NullVoxPopuli]

👍

[beauby]

Anybody interested in adding a note for each adapter in docs/general/adapters.md?