After changing the faq_url the Guidelines are called FAQ again in the sidebar. And the about page now mentions both Guidelines and FAQ.
Edit: I see the reversion has to do with translations. I renamed all instances of FAQ in site_texts, and that fixed that. But the now duplicate Guidelines on the about page remains.
(In my case, i moved the guidelines to the category set for the Doc categories plugin. The link works as it should.)
Also, i think the faq_url setting in admin could mention Guidelines in the description for clarity and searchability. (possibly also a translation)