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

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

Errors found:
	Errors in parameters section
		Parameter "flags" description should start with capital letter
```
`Achieved through PyData 2018 pandas sprint.`
Unsure best way to phrase `flags` parameter as description starts with the ```re``` module. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20870?src=pr&el=h1) Report
> Merging [#20870](https://codecov.io/gh/pandas-dev/pandas/pull/20870?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c4da79b5b322c73d8e61d1cb98ac4ab1e2438b40?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20870/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20870?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20870      +/-   ##
==========================================
+ Coverage   91.79%    91.8%   +<.01%     
==========================================
  Files         153      153              
  Lines       49411    49411              
==========================================
+ Hits        45359    45361       +2     
+ Misses       4052     4050       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.19% <ø> (ø)` | :arrow_up: |
| #single | `41.91% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20870?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20870/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.34% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20870/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.59% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20870?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/20870?src=pr&el=footer). Last update [c4da79b...99eb015](https://codecov.io/gh/pandas-dev/pandas/pull/20870?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Hey @WillAyd I've tried to fix your correction. What's the next step? Thanks @Blair-Young! Thanks @TomAugspurger looking forward to more.