[Feat] example about operators in exp class

In the `exp` class,
 https://github.com/BernieHuang2008/MercurySQL/blob/main/MercurySQL/core.py#L855

We can give out some example usages for the operators, such as 
```python
def __sub___(...):
"""
.. codeblock
   a - b
"""
```

Something like that.