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
