Draft: [EmberData] Update guides to suggest include to be an array

[Baltazore]

As part of discussion in https://github.com/emberjs/data/issues/9502 we found that there was a long-lasting miss-understanding of include param type definition. As more and more people using TS, we need to fix it, as the type for include would be array only, not a string.

So this PR aims to correct guides and docs to suggest user to use include value as an array.