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. :)
```