fix golint issues in core/contextx (#477)

core/contextx/deadline.go

@@ -5,6 +5,8 @@ import (
 	"time"
 )
 
+// ShrinkDeadline returns a new Context with proper deadline base on the given ctx and timeout.
+// And returns a cancel function as well.
 func ShrinkDeadline(ctx context.Context, timeout time.Duration) (context.Context, func()) {
 	if deadline, ok := ctx.Deadline(); ok {
 		leftTime := time.Until(deadline)

core/contextx/unmarshaler.go

@@ -19,6 +19,7 @@ func (cv contextValuer) Value(key string) (interface{}, bool) {
 	return v, v != nil
 }
 
+// For unmarshals ctx into v.
 func For(ctx context.Context, v interface{}) error {
 	return unmarshaler.UnmarshalValuer(contextValuer{
 		Context: ctx,

core/contextx/valueonlycontext.go

@@ -21,6 +21,7 @@ func (valueOnlyContext) Err() error {
 	return nil
 }
 
+// ValueOnlyFrom takes all values from the given ctx, without deadline and error control.
 func ValueOnlyFrom(ctx context.Context) context.Context {
 	return valueOnlyContext{
 		Context: ctx,