Update post visibility docs

[diondiondion]

Changes proposed in this PR:

• Changes the post visibility docs (previously called "Publishing levels") to more accurately match current in-app terminology
• Removes confusing wording around permalinks and having to be logged-in to see posts, where appropriate (e.g. "Seeing your post at the permalink requires being logged in on the same website as someone who was mentioned")
• Updates post visibility screenshot to match current UI