Add explanatory comment fo filtersForEra function

cardano-api/test/cardano-api-test/Test/Cardano/Api/ProtocolParameters.hs

@@ -127,6 +127,11 @@ patchProtocolParamsJSONOrFail era b = maybe (fail "Cannot fix JSON") return $ pa
                                                   =<< replace "minCommitteeSize" "committeeMinSize"
                                                         (applyFilters filters o)
 
+        -- | Legacy ProtocolParams ToJSON renders all fields from all eras in all eras,
+        -- | because it is the same data type for every era. But this is not backwards compatible
+        -- | because it means that new eras can modify the fields in old eras. For this reason, when
+        -- | comparing to PParams we use this function to filter fields that don't belong to
+        -- | particular era we are testing.
         filtersForEra :: CardanoEra era -> Maybe [String]
         filtersForEra ShelleyEra = Just [ "collateralPercentage", "costModels", "executionUnitPrices"
                                         , "maxBlockExecutionUnits", "maxCollateralInputs", "maxTxExecutionUnits"