Deprecate visibility parameter value `private` and use `followers_only` instead

[ashwinvis]

Context

While trying to read the docs, I assumed private meant direct messages and got confused

Screenshot of the online docs: to see how it reads now

I think the problem was 2 fold

1. It was not formatted. I have this fixed in #362
2. The Mastodon web UI and the API docs mentions "followers_only" and not "private". This can be misleading.

I wonder if we can use followers_only as the visibility parameter value. We can still support private for a while with a warning.

[codl]

The Mastodon API does use private, and given that this library is a fairly direct wrapper over the API, I don't think changing the terms would be a good idea. The documentation you've linked is the end-user documentation, not the API documentation.

I'm all for formatting the docs better, I'd go one step further and mention user-facing terms in the documentation as well, something like

• 'direct' - post will be visible only to mentioned users, known in Mastodon's UI as "Mentioned users only"
• 'private' - post will be visible only to followers, known in Mastodon's UI as "Followers only"
• ...

[ashwinvis]

Done

[halcy]

closed by your PR! and thank you so much for that!