`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 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.

quchen / prettyprinter