Small auth documentation issue with restricted public timeline

[arittner]

Hello!

IMHO, the https://github.com/mastodon/documentation/blob/master/content/en/methods/timelines.md is slightly wrong with the information about the required access to the public timeline, if public preview is disabled.

With a disabled preview, a user authentication is needed (see https://github.com/mastodon/mastodon/pull/11802/files). I checked it with an instance and I got (with Mastodon.py):

422, 'Unprocessable Entity', 'This method requires an authenticated user'

I checked is also with a registered application and I got again the same exception.

Maybe the documentation should consider this and note that a User authentication is needed.