DOC: Add numeric_only to DataFrame.quantile
- [x] closes #18608
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

My confusion with #18608 was primarily due to the `numeric_only` not being specified in the docs. Additional we do have tests covering this case in `pandas/tests/frame/test_quantile.py`
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21214?src=pr&el=h1) Report
> Merging [#21214](https://codecov.io/gh/pandas-dev/pandas/pull/21214?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6abb6148c8dc14e6279f31f2e620ecf52822107?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21214/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/21214?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21214   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49505    49505           
=======================================
  Hits        45466    45466           
  Misses       4039     4039
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (ø)` | :arrow_up: |
| #single | `41.88% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21214?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/21214/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.22% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21214?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/21214?src=pr&el=footer). Last update [f6abb61...d3bae9e](https://codecov.io/gh/pandas-dev/pandas/pull/21214?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke 