DOC: Fix to docstrings of is_type_factory and is_instance_factory

 - [X] closes #15485
 - [ ] tests added / passed
 - [ ] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15499?src=pr&el=h1) Report
> Merging [#15499](https://codecov.io/gh/pandas-dev/pandas/pull/15499?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/81c57e20da278494dfebc2f1043f5ff361a234f3?src=pr&el=desc) will **decrease** coverage by `-0.01%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15499      +/-   ##
==========================================
- Coverage   90.36%   90.36%   -0.01%     
==========================================
  Files         136      136              
  Lines       49532    49532              
==========================================
- Hits        44760    44759       -1     
- Misses       4772     4773       +1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15499?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/config.py](https://codecov.io/gh/pandas-dev/pandas/compare/81c57e20da278494dfebc2f1043f5ff361a234f3...678195f1c78e56ec173884b2f0706ab422b94fb5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29uZmlnLnB5) | `88.09% <ø> (ø)` | :white_check_mark: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/81c57e20da278494dfebc2f1043f5ff361a234f3...678195f1c78e56ec173884b2f0706ab422b94fb5?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.02% <ø> (-0.34%)` | :x: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15499?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/15499?src=pr&el=footer). Last update [81c57e2...678195f](https://codecov.io/gh/pandas-dev/pandas/compare/81c57e20da278494dfebc2f1043f5ff361a234f3...678195f1c78e56ec173884b2f0706ab422b94fb5?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). thanks!