DOC: Fix docstring for tsplot
Browsing through pandas, I noticed a small plotting function who's docs were incomplete / out of date. This is a small update to the docstring for tsplot in plotting/_timeseries. 

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=h1) Report
> Merging [#17814](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7db7f82ed34dfe9b9768f2449b291d5abd6ef60a?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17814/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17814      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49978    49978              
==========================================
- Hits        45611    45602       -9     
- Misses       4367     4376       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `60.73% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17814?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/17814?src=pr&el=footer). Last update [7db7f82...55bd4a5](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=h1) Report
> Merging [#17814](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7db7f82ed34dfe9b9768f2449b291d5abd6ef60a?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17814/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17814      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.02%     
==========================================
  Files         163      163              
  Lines       49978    49980       +2     
==========================================
- Hits        45611    45604       -7     
- Misses       4367     4376       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `60.73% <ø> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `91.99% <0%> (-0.05%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.58% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/pivot.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9waXZvdC5weQ==) | `96.38% <0%> (+0.02%)` | :arrow_up: |
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `97.09% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17814?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/17814?src=pr&el=footer). Last update [7db7f82...5326533](https://codecov.io/gh/pandas-dev/pandas/pull/17814?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger :
Yep, I think you're right, sorry about that. Fixed! Not that I don't want better docstrings (@TomDonogue thanks for contributing on that part!), but shouldn't this function rather be removed/deprecated? 

We don't use it internally, and it is not exposed in the `pandas.plotting` namespace (it *is* still exposed in `pandas.tseries.plotting`). 
And I also don't think it is actually a useful function for users? (they would just do `ts.plot()`, and here you have to awkwardly provide an actual plotting function) @jorisvandenbossche 
I was basically poking through pandas, looking for an easy initial PR, so other than noticing that this function is exposed through ts.plotting, I don't have much experience with it. I'm not super clear on the use case, and whether it's all that useful. I tested it out, but it does seem like an awkward function, given you have to pass in a plotting function. 

Point being, if it's better to deprecate, that makes sense to me - let me know if there is anything I can / should do on my end. 

 @TomDonoghue can you deprecate this function? add a test for the deprecation & mention in the whatsnew for 0.22 closing as stale. if you want to re-visit this pls ping. @TomDonoghue Still welcome to do a PR to deprecate the function instead! (opened https://github.com/pandas-dev/pandas/issues/18627 for this)