feat(events): add information on X-Forwarded-For header (#907)

Describe the event structure if a reverse proxy is involved.

Author: bt90

events/loginattempt.rst

@@ -3,17 +3,36 @@ LoginAttempt
 
 When authentication is enabled for the GUI, this event is emitted on every
 login attempt. If either the username or password are incorrect, ``success``
-is false and in any case the given username is returned.  The included
-remote address concerns the immediate connecting host, which may not be the
-origin of the request, but e.g. a reverse proxy.
+is false and in any case the given username is returned.
 
 .. code-block:: json
 
     {
        "id" : 187,
        "time" : "2017-03-07T00:19:24.420386143+01:00",
        "data" : {
-          "remoteAddress" : "127.0.0.1:55530",
+          "remoteAddress" : "127.0.0.1",
+          "username" : "somename",
+          "success" : false
+       },
+       "type" : "LoginAttempt",
+       "globalID" : 195
+    }
+
+If the ``X-Forwared-For`` header is present and the connecting host is
+either on ``localhost`` or on the same LAN, it will be treated as a reverse
+proxy. In this case, the ``remoteAddress`` field is filled with the leftmost
+IP address from the header, and the additional ``proxy`` field retains the
+original IP of the connecting host.
+
+.. code-block:: json
+
+    {
+       "id" : 187,
+       "time" : "2017-03-07T00:19:24.420386143+01:00",
+       "data" : {
+          "proxy" : "127.0.0.1",
+          "remoteAddress" : "192.168.178.10",
           "username" : "somename",
           "success" : false
        },