I don't think code quality and documentation quality are strongly correlated, they are just complementary.

I'd say without documentation any code loses quality.

I'm not talking about giving a manual for every piece of code you write, but any code that will be used by others (specially in big companies) should be well documented.

If good code is not documented, it will probably cause the same amount of trouble for people using it, than a bad piece of code.

People often forget that their code normally ends up outliving them.

People also often forget that a lot of code doesn't survive the week, which is common in an agile setting. Documenting code is something you do when you know it can live for awhile.

Also, if the focus is on maintainability, the person documenting the code shouldn't be the same person who wrote it, who already has biases on what is obvious or not. Rather, it should be someone with access to the author(s) who can ask questions and document what they didn't get right away.