Undo introducing straight up incorrect documentation

[Shnatsel]

Undo introducing misleading documentation, following the clarifications from miniz_oxide author in #430 (comment)

I think it is still desirable to have some support for miniz_oxide level 10, but doing it properly will require code changes and not just documentation. For example, letting the API user set 10 and automatically lowering it to 9 when using one of the zlib backends, so that Cargo feature unification wouldn't outright break people's code. That will need some API design work, so I want to get the misleading docs out of the way ASAP. Sorry 😓

[Byron]

Thank you!

No worries about it - this happens, and in the end, it's me who should have done a better review as well.