关于规约TransactionMustHaveRollbackRule

## 规约原文

方法【%s】需要在Transactional注解指定rollbackFor或者在方法中显式的rollback

## 问题描述

https://github.com/alibaba/p3c/blob/master/p3c-pmd/src/main/resources/rulesets/java/ali-exception.xml
Positive example 2：
```
/**
* @author caikang
* @date 2017/04/07
*/
@Service
public class UserServiceImpl implements UserService {
@Override
@Transactional(rollbackFor = Exception.class)
public void save(User user) {
//some code
//db operation
}
}
```
以上例子中，save方法并不会产生Checked Exception，没必要加rollbackFor吧？

## 修改建议

当方法抛出Checked Exception时再触发

Start with the ali-exception.xml ruleset linked in the issue and locate TransactionMustHaveRollbackRule, then compare its handling of methods with checked and unchecked exceptions. Confirm the existing Positive example 2 behavior and inspect nearby rule tests if available. Done means the rule requires rollbackFor or explicit rollback only when a checked exception can be thrown.

Written by the indexing model from the issue text.