This fixes gitlab-org/gitlab-ce#49953, as noted in the documentation
this feature is intended to be used when SSH certificates are
enabled. Then this warning becomes not only pointless, but also
misleading.

This builds on top of gitlab-org/gitlab-ce!21009 since both need to
modify the same documentation, which avoids a merge conflict.

See also the gitlab-org/gitlab-ce#49218 issue and associated merge
request.