docs(proxy): say TRUST_CF_CONNECTING_IP needs a Cloudflare-only origin

The flag makes the app believe CF-Connecting-IP from every connection,
before the peer check and whatever TRUST_PROXY_HOPS says, which is safe
only when nothing but Cloudflare can reach the origin. The reverse-proxy
page now says so with the ways to get there, the compose comment and both
example files carry the same condition, and the variable is in the env
manifest. The page's list of peers whose forwarding headers are read now
matches the code: loopback, 0.0.0.0/8, private, CGNAT, link-local,
unique-local and site-local, the reserved blocks and multicast. Those last
two stay trusted, since no internet host can connect from them; a test
pins every listed range and the Cloudflare precedence.

.env.example

@@ -202,7 +202,10 @@ API_TOKEN_HMAC_KEY="REPLACE-WITH-OUTPUT-OF-openssl-rand-hex-32"
 #
 # Set this to "1" ONLY when every public request to HealthLog terminates
 # at Cloudflare (Tunnel, Workers, classic proxy) before it reaches your
-# origin. Deployments behind nginx, Caddy, Traefik, an AWS ALB, or any
+# origin, and the origin accepts connections from Cloudflare alone (a
+# Tunnel, or a firewall that admits only Cloudflare's address ranges): the
+# header is believed from every connection, before any other check.
+# Deployments behind nginx, Caddy, Traefik, an AWS ALB, or any
 # direct-to-Next.js exposure MUST leave this off.
 #
 # Security implications when the flag is mis-set:
@@ -224,7 +227,10 @@ API_TOKEN_HMAC_KEY="REPLACE-WITH-OUTPUT-OF-openssl-rand-hex-32"
 # Number of trusted reverse-proxy hops in front of the app. The client IP is
 # read that many entries from the right of the X-Forwarded-For chain. Set 0
 # for a directly-exposed host; 1 (the default when unset) behind a single
-# reverse proxy. X-Real-IP is read only at 1. A wrong value makes the app
+# reverse proxy. X-Real-IP is read only at 1. Forwarding headers are read only
+# from a connection whose own address is not public (loopback, private, CGNAT,
+# unique-local, link-local, reserved or multicast; the full list is in
+# docs/self-hosting/reverse-proxy.md). A wrong value makes the app
 # refuse the headers, log one warning, and share one rate-limit bucket across
 # all anonymous callers.
 # TRUST_PROXY_HOPS="1"