Provide a reference for the cost model keys

So that the CDDL spec can reference it.

closes #2634

eras/alonzo/test-suite/cddl-files/alonzo.cddl

@@ -311,6 +311,11 @@ language = 0 ; Plutus v1
 
 costmdls = { * language => cost_model } ; New
 
+; The keys to the cost model map are not present in the serialization.
+; The values in the serialization are assumed to be ordered
+; lexicographically by their correpsonding key value.
+; The key values can be found in the Plutus repository:
+;   https://github.com/input-output-hk/plutus/blob/748a0c62de00f9944100b616367d9b6549346022/plutus-core/plutus-core/test/CostModelInterface/defaultCostModelParams.json
 cost_model = [ 166*166 int ] ; New
 
 transaction_metadatum =