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. 
