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).
CI failures look unrelated? thanks @kylekeppler  Owee, I'm MrMeeseeks, Look at me.

There seem to be a conflict, please backport manually. Here are approximate instructions:

1. Checkout backport branch and update it.

```
$ git checkout 1.2.x
$ git pull
```

2. Cherry pick the first parent branch of the this PR on top of the older branch:
```
$ git cherry-pick -m1 89b3d6b201b5d429a202b5239054d5a70c8b5071
```

3. You will likely have some merge/cherry-pick conflict here, fix them and commit:

```
$ git commit -am 'Backport PR #38471: DOC: fixes for assert_frame_equal check_freq argument'
```

4. Push to a named branch :

```
git push YOURFORK 1.2.x:auto-backport-of-pr-38471-on-1.2.x
```

5. Create a PR against branch 1.2.x, I would have named this PR:

> "Backport PR #38471 on branch 1.2.x"

And apply the correct labels and milestones.

Congratulation you did some good work ! Hopefully your backport PR will be tested by the continuous integration and merged soon!

If these instruction are inaccurate, feel free to [suggest an improvement](https://github.com/MeeseeksBox/MeeseeksDev).
                