DOC: Clarify and add fill_value example in arithmetic ops
- [ ] closes #19653
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Hello @HagaiHargil! Thanks for updating the PR.



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

##### Comment last updated on February 21, 2018 at 06:55 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19675?src=pr&el=h1) Report
> Merging [#19675](https://codecov.io/gh/pandas-dev/pandas/pull/19675?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d9551c8ee2a09ccc4c39d2b661f199334162edb6?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19675/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19675?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19675      +/-   ##
==========================================
+ Coverage   91.57%   91.61%   +0.03%     
==========================================
  Files         150      150              
  Lines       48817    48882      +65     
==========================================
+ Hits        44704    44782      +78     
+ Misses       4113     4100      -13
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <100%> (+0.03%)` | :arrow_up: |
| #single | `41.79% <100%> (+0.06%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19675?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `96.86% <100%> (+0.03%)` | :arrow_up: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `60% <0%> (-0.61%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.46% <0%> (-0.11%)` | :arrow_down: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.25% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `95.06% <0%> (-0.01%)` | :arrow_down: |
| [pandas/core/sparse/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL2ZyYW1lLnB5) | `94.81% <0%> (ø)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.2% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexes/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hcGkucHk=) | `98.78% <0%> (ø)` | :arrow_up: |
| [pandas/io/parquet.py](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnF1ZXQucHk=) | `71.79% <0%> (ø)` | :arrow_up: |
| ... and [14 more](https://codecov.io/gh/pandas-dev/pandas/pull/19675/diff?src=pr&el=tree-more) | |

------

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> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19675?src=pr&el=footer). Last update [d9551c8...3192301](https://codecov.io/gh/pandas-dev/pandas/pull/19675?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I'm also not sure what a shared docstring really is in this context :) I changed the confusing "array" into series\dataframe.

Regarding the "this is still not very clear" comment - 

First, I guess you agree with me that the previous text was plain wrong. Now I'm trying to find a clear enough sentence that fits the actual behavior of this keyword, but inconsistent behavior leads to badly-phrased docs. I'm really not sure what to say anymore. Looks like there are some linting errors failing the build: https://github.com/pandas-dev/pandas/pull/19675#issuecomment-365283811

This also seems to cause issues with our `_make_flex_doc` helper. Are you able to import pandas after making your changes locally? It was an interesting bug with the `{}` dict constructor. thanks @HagaiHargil 