Bluesky: update docs

[snarfed]

• profile setup
• !no-unauthenticated opt out
• links
• @-mentions
• hashtags
• 300 char limit, other limits, truncating
• fediverse opt in/out prompt
• reply restrictions
• flags <=> reports
• polling delay
• can't see @[email protected] user to opt in if your instance blocks us
• ...and DMs are in a "filtered notifications" section if your instance limits us: ActivityPub: do we need to do anything for instances that limit us? #972 (comment)