docs: Clarify that trust of X-Fowarded-Proto is also necessary.

Previously, `X-Forwarded-Proto` did not need to be set, and failure to
set `loadbalancer.ips` would merely result in bad IP-address
rate-limiting and incorrect access logs; after 0935d388f0, however,
failure to do either of those, if Zulip is deployed with `http_only`,
will lead to infinite redirect loops after login.  These are
accompanied by a misleading error, from Tornado, of:

    Forbidden (Origin checking failed - https://zulip.example.com does not match any trusted origins.): /json/events

This is most common with Docker deployments, where deployments use
another docker container, such as nginx or Traefik, to do SSL
termination.  See zulip/docker-zulip#403.

Update the documentation to reinforce that `loadbalancer.ips` also
controls trust of `X-Forwarded-Proto`, and that failure to set it will
cause the application to not function correctly.

docs/overview/changelog.md

@@ -192,14 +192,15 @@ log][commit-log] for an up-to-date list of all changes.
 - PostgreSQL 11 is no longer supported; if you are currently using it, you will
   need to [upgrade PostgreSQL](../production/upgrade.md#upgrading-postgresql)
   before upgrading Zulip.
-- Installations that deploy Zulip behind a [reverse
-  proxy][reverse-proxy-docs] should make sure the proxy is configured
-  to set the `X-Forwarded-Proto` HTTP header; the documentation has
-  updated example configuration.
+- Installations that deploy Zulip behind a [reverse proxy][reverse-proxy-docs]
+  should make sure the proxy is configured to set the `X-Forwarded-Proto` HTTP
+  header, and that [`loadbalancer.ips` is accurate][loadbalancer-ips] for the
+  reverse proxy's IP; the documentation has updated its example configurations.
 - Zulip's Twitter preview integration has been disabled due to Twitter
   desupporting the API that it relied on.
 
 [reverse-proxy-docs]: ../production/deployment.md#putting-the-zulip-application-behind-a-reverse-proxy
+[loadbalancer-ips]: ../production/deployment.md#configuring-zulip-to-trust-proxies
 
 ## Zulip 6.x series
 

docs/production/deployment.md

@@ -407,17 +407,22 @@ requests from a reverse proxy as follows:
 1. Finally, restart the Zulip server, using
    `/home/zulip/deployments/current/scripts/restart-server`.
 
+Note that Zulip must be able to accurately determine if its connection to the
+client was over HTTPS or not; if you enable `http_only`, it is very important
+that you correctly configure Zulip to trust the `X-Forwarded-Proto` header from
+its proxy (see the next section), or clients may see infinite redirects.
+
 #### Configuring Zulip to trust proxies
 
-Before placing Zulip behind a reverse proxy, it needs to be configured
-to trust the client IP addresses that the proxy reports via the
-`X-Forwarded-For` header. This is important to have accurate IP
-addresses in server logs, as well as in notification emails which are
-sent to end users. Zulip doesn't default to trusting all
-`X-Forwarded-For` headers, because doing so would allow clients to
-spoof any IP address; we specify which IP addresses are the Zulip
-server's incoming proxies, so we know how much of the
-`X-Forwarded-For` header to trust.
+Before placing Zulip behind a reverse proxy, it needs to be configured to trust
+the client IP addresses that the proxy reports via the `X-Forwarded-For` header,
+and the protocol reported by the `X-Forwarded-Proto` header. This is important
+to have accurate IP addresses in server logs, as well as in notification emails
+which are sent to end users. Zulip doesn't default to trusting all
+`X-Forwarded-*` headers, because doing so would allow clients to spoof any IP
+address, and claim connections were over a secure connection when they were not;
+we specify which IP addresses are the Zulip server's incoming proxies, so we
+know which `X-Forwarded-*` headers to trust.
 
 1. Determine the IP addresses of all reverse proxies you are setting up, as seen
    from the Zulip host. Depending on your network setup, these may not be the
@@ -956,8 +961,8 @@ which have more than 20k users.
 #### `ips`
 
 Comma-separated list of IP addresses or netmasks of external load balancers
-whose `X-Forwarded-For` should be respected. These can be individual IP
-addresses, or CIDR IP address ranges.
+whose `X-Forwarded-For` and `X-Forwarded-Proto` should be respected. These can
+be individual IP addresses, or CIDR IP address ranges.
 
 ### `[http_proxy]`