Propose CAP_NET_BIND_SERVICE instead running synapse with root

There are alternative ways to use low numbered ports besides root. Users might be misleaded thinking they should run synapse with root privileges.

Thanks to discussion with @bones_was_here.

Author: smoorg

docs/reverse_proxy.md

@@ -8,7 +8,7 @@ It is recommended to put a reverse proxy such as
 [relayd](https://man.openbsd.org/relayd.8) in front of Synapse. One advantage
 of doing so is that it means that you can expose the default https port
 (443) to Matrix clients without needing to run Synapse with root
-privileges.
+privileges or `CAP_NET_BIND_SERVICE` to bind such a low port number.
 
 You should configure your reverse proxy to forward requests to `/_matrix` or
 `/_synapse/client` to Synapse, and have it set the `X-Forwarded-For` and