It only talks about "$wgLogo" which has to be replaced with "$wgLogos" (i.e. the plural one) because it is not supported anymore and does not exist on new installations. I don't really understand the multiple logos thing so I can't do it.
Topic on Manual talk:FAQ
Please see the box on https://www.mediawiki.org/wiki/Manual_talk:FAQ which says "Please do not post support questions here."
I don't think it's a support question? I had no problem figuring some solution. It's a comment on the text on the wiki being outdated, participating on how to improve the text on this wiki. The second most recent post here is also about a case of Manual:FAQ containing information about an earlier MW version than what people are now going to install, so I wouldn't think of a different place to point this out.
Argh, you are right, I am sorry. Fell free to edit it, I'd say? :)
No worries :) Good to know, I *may* have a go at it if I at some point feel a bit more confident in the way $wgLogos works