The "view wiki pages" permission for guests has always applied to
private projects, the documentation was previously incorrect.

The documentation was originally changed in 9b12562c however the
:read_wiki permissions at that same ref were the same as they are now
(allowing guests of private projects to read the wiki).

We also have a feature
https://gitlab.com/gitlab-org/gitlab/issues/18739
that explicitly serves the wiki to guests of private projects, so this
further prooves that this documentation was wrong.

https://gitlab.com/gitlab-org/gitlab/issues/34761