DOC: Update the pandas.Series.dt.round/floor/ceil docstrings
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [ ] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

```
scripts/validate_docstrings.py pandas.Series.dt.floor  

Errors found:
	Summary does not start with capital
	No extended summary found
	Errors in parameters section
		Parameters {'args', 'kwargs'} not documented
		Unknown parameters {'freq'}
	See Also section not found
```
 Same errors are received from applying `validate_docstrings.py` to `pandas.Series.dt.ceil` and `pandas.Series.dt.round` as well.  

`args` and `kwargs` parameters are not the parameters that the clients are exposed to (`freq` is). The respective errors are received because of the way code is written with accessors.
  
We wanted to keep the docstrings of `round`, `floor` and `ceil` connected to keep the code as concise as possible. For that reason we also didn't add a customized extended summary for these methods. 

@TomAugspurger Thanks a lot for the review. I have fixed the issues you have pointed out. Hello @TomAugspurger, the checks are finalized. The AppVeyor test has failed. Any idea why this might be? I'm seeing that this test has failed for most of the PRs. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20187?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@e5e4ae9`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20187/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/20187?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20187   +/-   ##
=========================================
  Coverage          ?   91.72%           
=========================================
  Files             ?      150           
  Lines             ?    49162           
  Branches          ?        0           
=========================================
  Hits              ?    45096           
  Misses            ?     4066           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <100%> (?)` | |
| #single | `41.86% <100%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20187?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/20187/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `96.72% <100%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20187?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/20187?src=pr&el=footer). Last update [e5e4ae9...fcd3bdf](https://codecov.io/gh/pandas-dev/pandas/pull/20187?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think Jeff disabled appveyor for the sprint. Thanks!