DOC: fixes for assert_frame_equal check_freq argument
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

This is just a minor documentation change to fix the `versionadded` tag on the `check_freq` argument to `assert_frame_equal` and also to add a description of this backwards incompatible chage to the 1.1.0 Whats New (as recommended in https://github.com/pandas-dev/pandas/issues/35570#issuecomment-674064533).