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.

scripts/env-manifest.json

@@ -424,7 +424,12 @@
         },
         {
           "name": "TRUST_PROXY_HOPS",
-          "purpose": "Trusted reverse-proxy hop count; the client IP is read that many entries from the right of X-Forwarded-For.",
+          "purpose": "Trusted reverse-proxy hop count; the client IP is read that many entries from the right of X-Forwarded-For. Forwarding headers are read only from a peer whose own address is not public (loopback, private, CGNAT, unique-local, link-local, reserved, multicast).",
+          "example": "1"
+        },
+        {
+          "name": "TRUST_CF_CONNECTING_IP",
+          "purpose": "\"1\" takes the client IP from Cloudflare's CF-Connecting-IP header, from every connection and before the peer check. Safe only when the origin accepts connections from Cloudflare alone (a Tunnel, or a firewall admitting only Cloudflare's ranges); otherwise any caller forges the address rate limits and the audit log record. Off by default.",
           "example": "1"
         },
         {