id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
332077435,MDU6SXNzdWUzMzIwNzc0MzU=,2232,resample monthly to seasonal docstring example is wrong,6049169,closed,0,,,3,2018-06-13T16:45:48Z,2018-07-05T19:01:17Z,2018-07-05T19:01:17Z,CONTRIBUTOR,,,,"The doc string for `resample` contains an example for calculating a seasonal average from monthly values:

https://github.com/pydata/xarray/blob/9c8005937556211a8bf28a946744da3768846c5a/xarray/core/common.py#L620-L637

But running the code in the docstring gives different output:

```python
>>> da = xr.DataArray(np.linspace(0, 11, num=12),
                   coords=[pd.date_range('15/12/1999',
                           periods=12, freq=pd.DateOffset(months=1))],
                   dims='time')
>>> da
<xarray.DataArray (time: 12)>
array([ 0.,  1.,  2.,  3.,  4.,  5.,  6.,  7.,  8.,  9., 10., 11.])
Coordinates:
  * time     (time) datetime64[ns] 1999-12-15 2000-01-15 2000-02-15 ...

>>> da.resample(time=""Q-DEC"").mean()
<xarray.DataArray (time: 5)>
array([ 0. ,  2. ,  5. ,  8. , 10.5])
Coordinates:
  * time     (time) datetime64[ns] 1999-12-31 2000-03-31 2000-06-30 ...
```
#### Problem description

The difference is undesirable. Furthermore, the user is probably after seasonal averages computed for DJF, MAM, JJA, and SON, which is not what the example code produces. The example produces averages for JFM, AMJ, JAS, and OND.

Changing to

```
da.resample(time=""Q-NOV"").mean()
``` 
produces three month averages that align with the expected seasons.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

xarray: 0.10.2
pandas: 0.22.0
numpy: 1.14.2
scipy: 1.0.1
netCDF4: 1.3.1
h5netcdf: None
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.17.2
distributed: 1.21.5
matplotlib: 2.2.2
cartopy: 0.16.0
seaborn: 0.8.1
setuptools: 38.4.0
pip: 10.0.1
conda: None
pytest: 3.3.2
IPython: 6.2.1
sphinx: 1.7.2
</details>
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