Document the behavior of `axis=None` with `style.background_gradient`
Add docstring entry for using `axis=None`with `style.background_gradient`, which was added in #21259. Also reword to be in numerical order and consistent with how `highlight_max` and `highligh_min` defines column-wise and row-wise.

- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=h1) Report
> Merging [#25551](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d10bbce349b293e9f753d799cd2ad032d5ec7333?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/25551/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25551      +/-   ##
==========================================
+ Coverage   91.75%   91.75%   +<.01%     
==========================================
  Files         173      173              
  Lines       52960    52960              
==========================================
+ Hits        48595    48596       +1     
+ Misses       4365     4364       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.33% <ø> (ø)` | :arrow_up: |
| #single | `41.71% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25551?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/25551?src=pr&el=footer). Last update [d10bbce...bf2d569](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=h1) Report
> Merging [#25551](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d10bbce349b293e9f753d799cd2ad032d5ec7333?src=pr&el=desc) will **decrease** coverage by `0.49%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25551/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##           master   #25551     +/-   ##
=========================================
- Coverage   91.75%   91.26%   -0.5%     
=========================================
  Files         173      173             
  Lines       52960    52966      +6     
=========================================
- Hits        48595    48338    -257     
- Misses       4365     4628    +263
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <ø> (-0.5%)` | :arrow_down: |
| #single | `41.71% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/style.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvc3R5bGUucHk=) | `96.69% <ø> (ø)` | :arrow_up: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `38.56% <0%> (-33.19%)` | :arrow_down: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `93.3% <0%> (-2.24%)` | :arrow_down: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `90.88% <0%> (-1.41%)` | :arrow_down: |
| [pandas/core/internals/managers.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL21hbmFnZXJzLnB5) | `93.92% <0%> (-0.92%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `93.65% <0%> (-0.51%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `91.63% <0%> (-0.14%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.79% <0%> (-0.06%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25551/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.66% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25551?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/25551?src=pr&el=footer). Last update [d10bbce...8b1c6f0](https://codecov.io/gh/pandas-dev/pandas/pull/25551?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @WillAyd Fixed. Sorry, didn't notice that the pandas doc used the stylized left/right unicode quotation marks when I copied from your suggestion. Thanks @joelostblom !