Clarify difference between visibility options

[sk22]

The visibility options (that currently lack "Unlisted", see #103) lack clarification (more so if "Unlisted" gets added).
It's not obvious that "Public" also makes toots visible in the public timelines (local and federated; even though the app currently only shows the local timeline). This might be a security risk, especially for marginalized people and especially on big instances. Users should know that posting "Public" toots will have their toots be seen by anyone on their instance, or in the whole federation (and thus have an option to post unlisted, see #103 (comment) and #103 (comment)).

A way to improve this could be to add small explanation texts to each visibility option, for example:

• Public
  Publicly visible on your profile and in public timelines
• Unlisted
  Publicly visible on your profile and in your followers' timelines
• Followers only
  Visible only to people who follow you
• Only people I mention
  Visible only to people mentioned in this post