blockchain (v2): async testing notes (#4484)

Add some inline notes for context of why test are commented out and not deleted.

Ref. #4482

blockchain/v2/reactor_test.go

@@ -165,6 +165,8 @@ func newTestReactor(p testReactorParams) *BlockchainReactor {
 	return r
 }
 
+// This test is left here and not deleted to retain the termination cases for
+// future improvement in [#4482](https://github.com/tendermint/tendermint/issues/4482).
 // func TestReactorTerminationScenarios(t *testing.T) {
 
 // 	config := cfg.ResetTestRoot("blockchain_reactor_v2_test")