Updates manual install doc with auth.domain certs instructions.

doc/example-config-files/prosody.cfg.lua.example

 	-- Not essential, but recommended
 		"private"; -- Private XML storage (for room bookmarks, etc.)
 		"vcard"; -- Allow users to set vCards
-	
+
 	-- These are commented by default as they have a performance impact
 		--"privacy"; -- Support privacy lists
 		"compression"; -- Stream compression (requires the lua-zlib package installed)
 
 	c2s_require_encryption = false
 
+VirtualHost "auth.jitsi.example.com"
+	ssl = {
+		key = "/var/lib/prosody/auth.jitsi.example.com.key";
+		certificate = "/var/lib/prosody/auth.jitsi.example.com.crt";
+	}
+	authentication = "internal_plain"
+
 ------ Components ------
 -- You can specify components to add hosts that provide special services,
 -- like multi-user conferences, and transports.

doc/manual-install.md

 - add domain with authentication for conference focus user:
 ```
 VirtualHost "auth.jitsi.example.com"
+    ssl = {
+        key = "/var/lib/prosody/auth.jitsi.example.com.key";
+        certificate = "/var/lib/prosody/auth.jitsi.example.com.crt";
+    }
     authentication = "internal_plain"
 ```
 - add focus user to server admins:
 Generate certs for the domain:
 ```sh
 prosodyctl cert generate jitsi.example.com
+prosodyctl cert generate auth.jitsi.example.com
+```
+
+Add auth.jitsi.example.com to the trusted certificates on the local machine:
+```sh
+ln -sf /var/lib/prosody/auth.jitsi.example.com.crt /usr/local/share/ca-certificates/auth.jitsi.example.com.crt
+update-ca-certificates
 ```
 
 Create conference focus user: