`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`.

Start with the README section describing prettyprinter-compat-ansi-wl-pprint and compare its drop-in replacement claims with the incompatibilities listed here. Update the documentation to explain the missing Pretty (Maybe a) instance and the annotation-polymorphism limitation, then verify that the limitations are clear to users of ansi-wl-pprint-dependent packages.

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