DOC: update the pandas.DateTimeArrayMixin.is_month_start docstring
- [x] closes #20146 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
should close PR #20146
Hello @CalmDownKarm! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/datetimes.py`](https://github.com/CalmDownKarm/pandas/blob/4a0c57270c968e8072d4b910ee930f59654711d1/pandas/core/arrays/datetimes.py) !



##### Comment last updated on October 11, 2018 at 07:18 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23051?src=pr&el=h1) Report
> Merging [#23051](https://codecov.io/gh/pandas-dev/pandas/pull/23051?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9285820d33d4986769a84f3db5f25cff8bf85d8d?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23051/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23051?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23051      +/-   ##
==========================================
+ Coverage   92.19%   92.22%   +0.03%     
==========================================
  Files         169      169              
  Lines       50954    51259     +305     
==========================================
+ Hits        46975    47275     +300     
- Misses       3979     3984       +5
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <100%> (+0.05%)` | :arrow_up: |
| #single | `42.23% <100%> (-0.04%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23051?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lcy5weQ==) | `97.46% <100%> (+0.08%)` | :arrow_up: |
| [pandas/io/feather\_format.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2ZlYXRoZXJfZm9ybWF0LnB5) | `77.14% <0%> (-8.58%)` | :arrow_down: |
| [pandas/core/arrays/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2RhdGV0aW1lbGlrZS5weQ==) | `93.21% <0%> (-1.69%)` | :arrow_down: |
| [pandas/core/dtypes/concat.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbmNhdC5weQ==) | `96.34% <0%> (-1.35%)` | :arrow_down: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.27% <0%> (-0.98%)` | :arrow_down: |
| [pandas/core/arrays/sparse.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3NwYXJzZS5weQ==) | `91.84% <0%> (-0.71%)` | :arrow_down: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.21% <0%> (-0.41%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `94.24% <0%> (-0.4%)` | :arrow_down: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `70.76% <0%> (-0.28%)` | :arrow_down: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.86% <0%> (-0.25%)` | :arrow_down: |
| ... and [55 more](https://codecov.io/gh/pandas-dev/pandas/pull/23051/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23051?src=pr&el=footer). Last update [9285820...9e0f242](https://codecov.io/gh/pandas-dev/pandas/pull/23051?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @datapythonista I made the changes you asked for, but both commands you asked me to run failed - AttributeError: module 'pandas' has no attribute 'DateTimeArrayMixin' probably a broken setup on my end? is the validation script happy now? seems like there are whitespaces at the end of some lines in your code (check travis), can you remove them and validate pep8 @datapythonista fixed flake8 and validation script works now, but I'm unable to generate the html. 

python scripts/validate_docstrings.py pandas.core.arrays.datetimes.DatetimeArrayMixin.is_month_start
validates fine
but if I run 
```python doc/make.py html --single=pandas.core.arrays.datetimes.DatetimeArrayMixin.is_month_start``` 
I get
AttributeError: module 'pandas.core.arrays' has no attribute 'datetimes'
 Thanks @CalmDownKarm !