`prettyprinter-compat-ansi-wl-pprint ` is not quite a drop-in replacement as claimed

This was noticed in https://github.com/ekmett/ansi-wl-pprint/pull/32#issue-1712218860, but I've found some other incompatibilities:

- There's no `Pretty a => Pretty (Maybe a)` instance, which old code may rely on.
- We have `Doc = Doc AnsiStyle`, which means that any existing `Pretty` instances which use basic functions like [`text`](https://hackage.haskell.org/package/ansi-wl-pprint-1.0.2/docs/Text-PrettyPrint-ANSI-Leijen.html#v:text) no longer compile, since `prettyprinter`'s `pretty` has to be annotation-polymorphic.

These are not easily solved, so I think the README ought to just be clearer about the limitations.

This has caused issues for some `ansi-wl-pprint`-dependent packages, such as `tmp-postgres`.

prettyprinter-compat-ansi-wl-pprint について説明している README のセクションから始め、そこにある drop-in replacement の主張と、ここに列挙されている非互換性を比較してください。ドキュメントを更新して、欠落している Pretty (Maybe a) インスタンスとアノテーション多相性の制限を説明し、その後、ansi-wl-pprint に依存するパッケージのユーザーにとって制限が明確であることを確認してください。

索引モデルが issue の本文から書いたものです。