DOC: Clarify that unique() promotes dtype to 64-bit
I found this behavior surprising:

```python
In [1]: pd.Series([1,2,3], dtype=np.uint8).unique()
Out[1]: array([1, 2, 3], dtype=uint64)
```
... because I did not expect a different dtype in the result.  This PR adds a sentence to the docs to clarify this.

- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- (n/a) closes #xxxx
- (n/a) tests added / passed
- (n/a) whatsnew entry

FWIW, These changes do pass `black` and `flake8`.  The CI failures appear unrelated. CI failure is fixed on master.

Is this deliberate, or an implementation detail? We should document that. >Is this deliberate, or an implementation detail?

My hunch is that it's an implementation detail.  For example, `pd.unique()` differs from `np.unique()` in this respect, and I don't know why pandas would go out of its way to be different.  It's probably just incidental.

Heck, maybe it's even accidental (i.e. a bug).  Should [this line][1] be changed as follows?

[1]: https://github.com/pandas-dev/pandas/blob/8993facf0897a192c3a933dea99d5c759959c16b/pandas/core/algorithms.py#L407

```diff
-    uniques = _reconstruct_data(uniques, dtype, original)
+    uniques = _reconstruct_data(uniques, original.dtype, original)
```

In case that looks right to you, I've opened a PR: #27874 superseded by #27874 