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.

yeah this is better. of course there is no visibility of this 'change' as it happened in the past, but now documented.

thanks @kylekeppler  @meeseeksdev backport 1.2.x Something went wrong ... Please have  a look at my logs.