Missing documentation and examples for passing an external parameter to a DQL upsert

[damonfeldman]

There are bits of code for mutations, upserts, external parameters (though external parameter documentation is still pretty light) but nothing that pulls it together for an external parameter to a DQL upsert.

A discuss article indicates that the external variables can only be used in the query portion of the mutation, so this should be clarified as well in the docs.