docs: expand documentation

Signed-off-by: Francisco de Borja Aranda Castillejo <borja@zetachain.com>

x/evm/keeper/state_transition.go

@@ -72,7 +72,11 @@ func (k *Keeper) NewEVM(
 	}
 
 	vmConfig := k.VMConfig(ctx, msg, cfg, tracer)
+
+	// rules are used to determine the active precompiles for the current block height.
+	// i.e rules.IsByzantium, rules.IsConstantinople, etc.
 	rules := cfg.ChainConfig.Rules(big.NewInt(ctx.BlockHeight()), cfg.ChainConfig.MergeNetsplitBlock != nil)
+
 	contracts := make(map[common.Address]vm.PrecompiledContract)
 	active := make([]common.Address, 0)
 
@@ -101,8 +105,8 @@ func (k *Keeper) NewEVM(
 	evm := vm.NewEVM(blockCtx, txCtx, stateDB, cfg.ChainConfig, vmConfig)
 
 	// Set the precompiled contracts:
-	// - contracts contains all the contracts.
-	// - active contains the addresses of the active contracts.
+	// - contracts contains all the precompiled contracts.
+	// - active contains *only* the addresses of the active precompiled contracts.
 	evm.WithPrecompiles(contracts, active)
 
 	return evm

x/evm/statedb/native.go

@@ -7,6 +7,8 @@ import (
 
 var _ JournalEntry = nativeChange{}
 
+// nativeChange represents a state change originated by an action executed by the EVM.
+// It has to implement JournalEntry interface, so it can be appended to the chain journal.
 type nativeChange struct {
 	snapshot types.MultiStore
 	events   int