Update visibility.md (#190)
Update visibility.md (#190)

Clarify the meaning of `hidden` and `invisible`. I'm still not sure `hidden` is the best class name, given its opposite meaning when used as the value for `visibility`, but hey ho.
quasarframework/quasar-framework.org,quasarframework/quasar-framework.org,rstoenescu/quasar-site,rstoenescu/quasar-site,quasarframework/quasar-framework.org,rstoenescu/quasar-site