DOC: Add user_guide examples and docstring example for df.plot.box and df.plot.hist
- [x] closes #42492
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

thanks @charlesdong1991 