DOC: update the DataFrame.sub docstring
Checklist for the pandas documentation sprint :

- [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 ##################################
################################################################################

Errors found:
	Use only one blank line to separate sections or paragraphs
	Errors in parameters section
		Wrong parameters order. Actual: ('other', 'axis', 'level', 'fill_value'). Documented: ('other', 'axis', 'fill_value', 'level')
		Parameter "other" has no description
		Parameter "axis" description should finish with "."
		Parameter "fill_value" description should finish with "."
		Parameter "level" description should finish with "."
	Missing description for See Also "DataFrame.rsub" reference

```

Validation script returns docstring style errors.

