refine comments for package core (#5014)

* Refine Comments

* refine comments for cache.go

* refine comments for log.go

* Update orm.go

* refine comments for orm_log.go,types.go

* Update utils.go

* Update doc.go

* refine comments

* refine comments

* Update db.go

* refine comments for core

core/utils/time.go

@@ -19,7 +19,7 @@ import (
 	"time"
 )
 
-// short string format
+// ToShortTimeFormat short string format
 func ToShortTimeFormat(d time.Duration) string {
 	u := uint64(d)
 	if u < uint64(time.Second) {