doc/schema: add default-func example for schema fields (#1127)

doc/md/schema-fields.md

@@ -233,6 +233,10 @@ func (User) Fields() []ent.Field {
 		field.Time("updated_at").
 			Default(time.Now).
 			UpdateDefault(time.Now),
+		field.String("name").
+			Default("unknown"),
+		field.String("cuid").
+			DefaultFunc(cuid.New),
 	}
 }
 ```