docs(netbird-proxy): document relay watchdog decision + health endpoint

Review follow-up: note in the compose template why the relay is not
covered by the proxy watchdog (fallback component, client-side relay
failover, restart: unless-stopped) and record the :9000/health endpoint
(verified on 0.73.2) for future active monitoring.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ansible/roles/netbird-proxy/templates/docker-compose.yml.j2

@@ -46,6 +46,13 @@ services:
   # must sit on the host's un-NATed public addresses — STUN behind NAT
   # reports wrong reflexive candidates. Both ports are >1024, so the
   # ip_unprivileged_port_start sysctl is not involved.
+  #
+  # Deliberately NOT covered by the netbird-proxy-watchdog (which probes the
+  # proxy's public URL): this relay is a fallback with client-side failover
+  # to the other relays, so a wedged instance degrades redundancy rather
+  # than availability; restart: unless-stopped catches crashes. If active
+  # relay monitoring is wanted later, the binary serves
+  # http://localhost:9000/health (verified on 0.73.2).
   relay:
     image: netbirdio/relay:{{ netbird_version }}
     container_name: netbird-relay