At the mentioned section of the documentation:
there's an anchor within the text "See the FAQ to enable logging output in Piwik and the various options" pointing to:
At that address there's no mention of logging output. Also, it should be within the FAQ pages, stating the description.
We think the correct destination should be the FAQ titled "I'm a developer, how do I enable logging (and / or SQL profiling) in Piwik?" at page:
Thanks, updated the page!