feat(tuning): add section for LAN-only (fixes #834, fixes #624)

[marbens-arch]

This adds a section to the tuning article for restricting network traffic to your local network only.

[acolomb]

I think you're missing some pieces where Internet facing communications take place. We already have somewhat of an overview here:

https://docs.syncthing.net/users/security.html

And it's probably good to align with this FAQ entry, or link from it to your new section:

https://docs.syncthing.net/users/faq.html#why-does-syncthing-connect-to-this-unknown-suspicious-address

[marbens-arch]

Wasn't thinking about usage reporting or crash reporting when I was writing this. But those should be included too.

[acolomb]

... and version upgrade checks as well?