DOC: clarify default behavior for categories ordering
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Just a doc clarification on default categories ordering, which can be pretty confusing when handling mixed dtypes (see also #22080 ).
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22081?src=pr&el=h1) Report
> Merging [#22081](https://codecov.io/gh/pandas-dev/pandas/pull/22081?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/26b3e7de56fdebaf1842b01d747a1a30a126c54a?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/22081/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22081?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22081   +/-   ##
=======================================
  Coverage   92.07%   92.07%           
=======================================
  Files         170      170           
  Lines       50688    50688           
=======================================
  Hits        46671    46671           
  Misses       4017     4017
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.48% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22081?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/22081/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `95.95% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22081?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/22081?src=pr&el=footer). Last update [26b3e7d...ddb03e4](https://codecov.io/gh/pandas-dev/pandas/pull/22081?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. @TomAugspurger @jorisvandenbossche wording look ok?