add clarification on no changes between 2019 and 2020
These tests are intended to validate that implementations are correctly generating output in accordance with the specification.
-Output was initially specified with draft 2019-09. It remained unchanged for draft 2020-12, but will receive an update with the next release.
+Output was initially specified with draft 2019-09. It remained largely unchanged for draft 2020-12, but will receive an update with the next release.
***NOTE** Although the formats didn't change between 2019-09 and 2020-12, the tests are replicated for 2020-12 because the `$schema` is different and implementations may (but shouldn't) produce different output.*
