DOC: update the NDFrame.transform docstring
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"
- [x] 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

Please include the output of the validation script below between the "```" ticks:

```
Docstring for "pandas.DataFrame.transform" correct. :)
```



Hello @oscar6echo! Thanks for updating the PR.

 - In the file [`pandas/core/generic.py`](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py), following are the PEP8 issues :

> [Line 8624:8](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py#L8624): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace
> [Line 8638:8](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py#L8638): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace
> [Line 8643:8](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py#L8643): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace
> [Line 8657:22](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py#L8657): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace
> [Line 8666:44](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py#L8666): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace
> [Line 8678:8](https://github.com/pandas-dev/pandas/blob/e1883f26c9f5e67a243b481d947cb83ea328f264/pandas/core/generic.py#L8678): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace



##### Comment last updated on April 12, 2018 at 21:37 Hours UTC We did this doc work with @DandanW924 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20200?src=pr&el=h1) Report
> Merging [#20200](https://codecov.io/gh/pandas-dev/pandas/pull/20200?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cdfce2b0ad99f7faad57cc5247cf33aab5725bed?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20200/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20200?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20200      +/-   ##
==========================================
- Coverage   91.82%    91.8%   -0.03%     
==========================================
  Files         152      152              
  Lines       49248    49210      -38     
==========================================
- Hits        45222    45176      -46     
- Misses       4026     4034       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <100%> (-0.03%)` | :arrow_down: |
| #single | `41.85% <45.45%> (-0.05%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20200?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.86% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `72.72% <0%> (-13.64%)` | :arrow_down: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `80.64% <0%> (-3.51%)` | :arrow_down: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `68.77% <0%> (-1.27%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.95% <0%> (-0.58%)` | :arrow_down: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.14% <0%> (-0.42%)` | :arrow_down: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.27% <0%> (-0.24%)` | :arrow_down: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.32% <0%> (-0.13%)` | :arrow_down: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.05% <0%> (-0.09%)` | :arrow_down: |
| [pandas/core/algorithms.py](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYWxnb3JpdGhtcy5weQ==) | `94.29% <0%> (-0.07%)` | :arrow_down: |
| ... and [19 more](https://codecov.io/gh/pandas-dev/pandas/pull/20200/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20200?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/20200?src=pr&el=footer). Last update [cdfce2b...e1883f2](https://codecov.io/gh/pandas-dev/pandas/pull/20200?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Finally, do you plan to merge it or abandon it ? Are there other changes required ? Pls advise.