DOC: Fixing 'a la' confusion in series.quantile documentation
Just stating that numpy.percentile also offers something similar to this

- [x] closes #21292
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] Just clarified quantile doc a bit

Hello @uds5501! Thanks for updating the PR.



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

##### Comment last updated on June 06, 2018 at 06:04 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21293?src=pr&el=h1) Report
> Merging [#21293](https://codecov.io/gh/pandas-dev/pandas/pull/21293?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4274b840e64374a39a0285c2174968588753ec35?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/21293/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21293?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21293      +/-   ##
==========================================
+ Coverage   91.85%   91.85%   +<.01%     
==========================================
  Files         153      153              
  Lines       49546    49549       +3     
==========================================
+ Hits        45509    45512       +3     
  Misses       4037     4037
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.25% <ø> (ø)` | :arrow_up: |
| #single | `41.87% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21293?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/21293/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.12% <ø> (ø)` | :arrow_up: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/21293/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.14% <0%> (+0.01%)` | :arrow_up: |
| [pandas/core/indexes/interval.py](https://codecov.io/gh/pandas-dev/pandas/pull/21293/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9pbnRlcnZhbC5weQ==) | `93.16% <0%> (+0.02%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21293?src=pr&el=footer). Last update [4274b84...194c5c1](https://codecov.io/gh/pandas-dev/pandas/pull/21293?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @uds5501 general feedback: can you give the PR a more informative title? (you can still edit the title, see 'edit' button at the right of it) @WillAyd  Did the update you asked for. Now numpy.percentile is included in See also section @uds5501 Thanks a lot!