Collections: make the adaptor structure a bit clearer

[josephjclark]

In the main docs, in the big info boxout, we should make it clearer that you do not choose the Collections adaptor, the API is always there. Maybe recommend choosing common by default.

In the Adaptor Overview docs, make it clear that the collections API can be used with any adaptor. Make it clear that you don't choose the Collections adaptor. Recommend choosing common.

Basically we need to tell a simpler story about how you use the collections API (its sort of an adaptor but not)