Update configuration doc to include socket@server

 `protocol=socket` and `socket=` path in [server]

fixes https://github.com/grafana/grafana/issues/8701

docs/sources/installation/configuration.md

@@ -127,10 +127,13 @@ Another way is put a webserver like Nginx or Apache in front of Grafana and have
 
 ### protocol
 
-`http` or `https`
+`http`,`https` or `socket`
 
 > **Note** Grafana versions earlier than 3.0 are vulnerable to [POODLE](https://en.wikipedia.org/wiki/POODLE). So we strongly recommend to upgrade to 3.x or use a reverse proxy for ssl termination.
 
+### socket
+Path where the socket should be created in case of `protocol=socket`. Please make sure that Grafana has appropiate permissions.
+
 ### domain
 
 This setting is only used in as a part of the `root_url` setting (see below). Important if you