document WithCache function

pkg/bbgo/cache.go

@@ -10,12 +10,19 @@ import (
 	"github.com/pkg/errors"
 )
 
-func WithCache(key string, obj interface{}, do func() (interface{}, error)) error {
+
+type DataFetcher func() (interface{}, error)
+
+// WithCache let you use the cache with the given cache key, variable reference and your data fetcher,
+// The key must be an unique ID.
+// obj is the pointer of your local variable
+// fetcher is the closure that will fetch your remote data or some slow operation.
+func WithCache(key string, obj interface{}, fetcher DataFetcher) error {
 	cacheDir := CacheDir()
 	cacheFile := path.Join(cacheDir, key+ ".json")
 
 	if _, err := os.Stat(cacheFile); os.IsNotExist(err) {
-		data, err := do()
+		data, err := fetcher()
 		if err != nil {
 			return err
 		}