DOC: Improve the docstring of DataFrame.nlargest
Co-authored-by: Igor C. A. de Lima <igorcadelima@gmail.com>
Signed-off-by: Carlos Eduardo Moreira dos Santos <cems@cemshost.com.br>
Signed-off-by: Igor C. A. de Lima <igorcadelima@gmail.com>

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:

```
################################################################################
################################## Validation ##################################
################################################################################

Docstring for "pandas.core.frame.DataFrame.nlargest" correct. :)
```
Hello @cemsbr! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on March 17, 2018 at 10:28 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20255?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@fb556ed`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `96.77%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20255/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20255?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20255   +/-   ##
=========================================
  Coverage          ?   91.72%           
=========================================
  Files             ?      150           
  Lines             ?    49165           
  Branches          ?        0           
=========================================
  Hits              ?    45099           
  Misses            ?     4066           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <96.77%> (?)` | |
| #single | `41.86% <9.67%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20255?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20255/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <96.77%> (ø)` | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20255?src=pr&el=footer). Last update [fb556ed...7dd4f02](https://codecov.io/gh/pandas-dev/pandas/pull/20255?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 As suggested, the changes result in the expected validation error below.

```
################################################################################
################################## Validation ##################################
################################################################################

Errors found:
        Errors in parameters section
                Parameter "keep" description should finish with "."
``` @cemsbr Thanks a lot :+1: 