DOC: update Series.combine/DataFrame.combine doc strings
This PR updates ``Series.combine``/``DataFrame.combine`` doc strings. In particular, it adds some short examples to the doc strings.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=h1) Report
> Merging [#18268](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7495e9ade5a0ca0c24611f94b6d6c294dbd5446e?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18268/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18268      +/-   ##
==========================================
- Coverage    91.4%   91.38%   -0.02%     
==========================================
  Files         164      164              
  Lines       49883    49878       -5     
==========================================
- Hits        45595    45581      -14     
- Misses       4288     4297       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.19% <ø> (-0.01%)` | :arrow_down: |
| #single | `39.41% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95% <ø> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.8% <ø> (-0.1%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/indexes/multi.py](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9tdWx0aS5weQ==) | `96.38% <0%> (-0.02%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18268?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/18268?src=pr&el=footer). Last update [7495e9a...94b1a12](https://codecov.io/gh/pandas-dev/pandas/pull/18268?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!