Clear up confusion from #19411
Modify docstring for pandas.util.testing.assert_frame_equal
closes #19411 
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=h1) Report
> Merging [#19554](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d24a9507ba539f455d9b90885edf098c7dc93e99?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19554/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19554   +/-   ##
=======================================
  Coverage   91.93%   91.93%           
=======================================
  Files         159      159           
  Lines       49733    49733           
=======================================
  Hits        45721    45721           
  Misses       4012     4012
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.31% <ø> (ø)` | :arrow_up: |
| #single | `42% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19554/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.33% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=footer). Last update [d24a950...32191bb](https://codecov.io/gh/pandas-dev/pandas/pull/19554?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @darcymeyer can you update? > can you update?

@TomAugspurger : Sure can 🙂  @jreback @TomAugspurger : All is green and rebased.  PTAL. thanks @gfyoung 