Add ref to docstring of Series:dtype
- [x] closes #24325 
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
Add ``:ref:`basics.dtypes` `` to the docstring of `dtype` of `pandas.Series`. 

@TomAugspurger Should we also add your examples to `basics.dtypes` ? 
```python
pd.Series([1, 2], dype=np.dtype("int64"))
pd.Series([1, 2], dtype=pd.api.types.CategoricalDtype([1, 2, 3]))
```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=h1) Report
> Merging [#24421](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/159772d0c9d49977d4b7f871c62c26e06890143a?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/24421/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24421   +/-   ##
=======================================
  Coverage    92.3%    92.3%           
=======================================
  Files         163      163           
  Lines       51950    51950           
=======================================
  Hits        47953    47953           
  Misses       3997     3997
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.71% <ø> (ø)` | :arrow_up: |
| #single | `43% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24421/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.71% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24421?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/24421?src=pr&el=footer). Last update [159772d...e63ed6e](https://codecov.io/gh/pandas-dev/pandas/pull/24421?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/24421?src=pr&el=h1) Report
> Merging [#24421](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/aa1549f7fd8ce81bf23296bf92fe9713aa8d6e20?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24421/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24421      +/-   ##
==========================================
- Coverage    92.3%   92.29%   -0.01%     
==========================================
  Files         163      163              
  Lines       51969    51969              
==========================================
- Hits        47968    47967       -1     
- Misses       4001     4002       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <ø> (ø)` | :arrow_up: |
| #single | `43.01% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/24421/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `93.72% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24421/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `87.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24421?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/24421?src=pr&el=footer). Last update [aa1549f...63c2aef](https://codecov.io/gh/pandas-dev/pandas/pull/24421?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @thoo 