DOC: move API breaking "check_freq" section from v1.2.1rst to v1.1.0.rst
and add reference to 1.1.0 whats new update in v1.2.1.rst

- [ ] closes #xxxx
- [ ] 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
- [x] whatsnew entry

This moves the doc notes for `assert_series_equal` and `assert_frame_equal` from the 1.2.1 whats new to the 1.1.0 whats new and adds a small reference to the section in the 1.1.0 page in the 1.2.1 page. This came up in #38471 and makes more sense to me than the current approach, but happy to close if current state is preferred.
