Add additional docs to Query Interface guide about composite primary key models [ci skip]

[adrianna-chang-shopify]

Added additional docs about ordering on the #first and #last methods for CPK models, as well as docs on the tuple-like syntax for hash conditions on the #where method.

I also mentioned the id_value method in the context of finder methods such as find_by and where, where usage with CPK models can be ambiguous. Open to feedback if there would be a better place to put this information. Does the example make sense? Is there value in highlighting that scenario?