Rename the --transport n0 value to relay and say what n0 means

"n0" is Number 0, the company behind iroh, and the name leaked from iroh's
own preset into this project's user interface, where it explains nothing.
The value is now --transport relay, which says what it does; n0 stays as an
accepted alias.

Also spells out, in the CLI help, the README, the threat model and the
TransportPolicy docs, whose infrastructure is involved: address records are
published to and resolved from dns.iroh.link, and the fallback relays are
Number 0's, in the US, EU and Asia-Pacific.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/threat-model.md

@@ -43,8 +43,9 @@ Read this before relying on anything here. The protocol is in
   handshake. Use `NetworkSecret::generate()`.
 - **Public address publication.** With `TransportPolicy::N0Defaults` or
   `DirectOnly`, iroh publishes a signed record of this endpoint's addresses,
-  keyed by its endpoint id, to Number 0's public pkarr/DNS service, and
-  resolves peers through it. The network secret is never published and
+  keyed by its endpoint id, to the public pkarr/DNS service run by Number 0
+  ("n0", the company behind iroh) at `dns.iroh.link`, and resolves peers
+  through it. The network secret is never published and
   membership cannot be inferred from a single record, but the endpoint's
   existence and its addresses become public. `LocalOnly` publishes nothing.
 - **Addresses and metadata are observable.** Anyone able to watch the network