Common docstring for ceil, floor and round
#### Code Sample, a copy-pastable example if possible

```
class TimelikeOps(object):
    """ common ops for TimedeltaIndex/DatetimeIndex, but not PeriodIndex """

    _round_doc = (
        """
        %s the index to the specified freq

        Parameters
        ----------
        freq : freq string/object

        Returns
        -------
        index of same type

        Raises
        ------
        ValueError
        if the freq cannot be converted
        """)

    def _round(self, freq, rounder):
        # round the local times
        values = _ensure_datetimelike_to_i8(self)
        result = round_ns(values, rounder, freq)
        result = self._maybe_mask_results(result, fill_value=NaT)

        attribs = self._get_attributes_dict()
        if 'freq' in attribs:
            attribs['freq'] = None
        if 'tz' in attribs:
            attribs['tz'] = None
        return self._ensure_localized(
            self._shallow_copy(result, **attribs))

    @Appender(_round_doc % "round")
    def round(self, freq, *args, **kwargs):
        return self._round(freq, np.round)

    @Appender(_round_doc % "floor")
    def floor(self, freq):
        return self._round(freq, np.floor)

    @Appender(_round_doc % "ceil")
    def ceil(self, freq):
        return self._round(freq, np.ceil)
```
#### Problem description
The DatetimeIndex.round(), ceil and floor share a common docstring which makes it difficult to edit such as adding see also, param section and examples. Can we make separate docstrings for these methods?
#### Expected Output

#### Output of ``pd.show_versions()``

<details>
INSTALLED VERSIONS
------------------
commit: 731d97164ba04150174c4e05f56d40f716044219
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-112-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_IN
LOCALE: en_US.UTF-8

pandas: 0.23.0.dev0+464.g731d971.dirty
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.4.0
Cython: 0.26
numpy: 1.13.1
scipy: 0.19.1
pyarrow: 0.8.0
xarray: 0.9.6
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: 0.7.9.None
psycopg2: None
jinja2: 2.9.6
s3fs: 0.1.2
fastparquet: 0.1.0
pandas_gbq: None
pandas_datareader: None
</details>
