DOC: Improve the docstring of String.str.zfill()
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Result of the Pandas sprint at PyData London 2018. Includes work by @owlas.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20864?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@876e568`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20864/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20864?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20864   +/-   ##
=========================================
  Coverage          ?   91.81%           
=========================================
  Files             ?      153           
  Lines             ?    49478           
  Branches          ?        0           
=========================================
  Hits              ?    45429           
  Misses            ?     4049           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <ø> (?)` | |
| #single | `41.85% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20864?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20864/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20864?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/20864?src=pr&el=footer). Last update [876e568...706688f](https://codecov.io/gh/pandas-dev/pandas/pull/20864?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hello @alkamid! Thanks for updating the PR.



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

##### Comment last updated on July 07, 2018 at 14:20 Hours UTC @datapythonista thanks a lot for your valuable feedback! I've addressed all your comments, does it look better to you now? @datapythonista thanks again, suggestions incorporated into PR. Let me know what you think now.