Update listening_ports documentation

Add description for optional port configuration

docs/UserManual.md

@@ -440,7 +440,9 @@ of 1000.
 ### listening\_ports `8080`
 Comma-separated list of ports to listen on. If the port is SSL, a
 letter `s` must be appended, for example, `80,443s` will open
-port 80 and port 443, and connections on port 443 will be SSL-ed.
+port 80 and port 443, and connections on port 443 will be SSL-ed. If the port
+should be optional the letter `o` must be appended, for example with `80o,443s`
+starting the server will not exit if binding to port 80 is not possible.
 For non-SSL ports, it is allowed to append letter `r`, meaning 'redirect'.
 Redirect ports will redirect all their traffic to the first configured
 SSL port. For example, if `listening_ports` is `80r,443s`, then all