docs(pong): require non-empty sni for Peer.sni and Property.sni

[VasilevNStas]

Description

Documents that Peer.sni and Property.sni MUST be non-empty and that clients MUST discard peers/properties with empty SNI rather than dialling them.

Problem

Peer.sni (field 6) had no non-empty contract. An empty SNI causes the client to send an empty SNI extension (or omit it entirely), which the target Node rejects. The resulting connection failure is indistinguishable from the Node being unreachable, so the client silently rotates away from a healthy Node or enters a retry loop.

Property.sni (field 8) already said "Empty while the slot is detached" but didn't instruct clients not to dial it.

Changes

• Peer.sni: added "MUST be non-empty; an empty sni would cause a TLS handshake failure indistinguishable from a dead Node. Clients MUST discard a Peer with an empty sni rather than dialling it."
• Property.sni: added "clients MUST NOT dial a slot with an empty sni."

Closes #20

[VasilevNStas]

@yegor256 @davvd plz review

[0crat]

@VasilevNStas Thanks for the contribution! You've earned +8 points for this: +16 as a basis; -8 for too few (8) hits-of-code. Please, keep them coming. Your running score is +36; don't forget to check your Zerocracy account too).