Expand SHOW DATABASES warning message

The message now states the impliciations of missing permissions. If the
MaxScale user does not have the permissions to view all databases, it will
only see its own databases.

server/modules/authenticator/MySQLAuth/dbusers.c

@@ -638,7 +638,8 @@ static bool check_server_permissions(SERVICE *service, SERVER* server,
 
             if (row && strcasecmp(row[0], "Y") != 0)
             {
-                MXS_WARNING("[%s] User '%s' is missing the SHOW DATABASES privilege.",
+                MXS_WARNING("[%s] User '%s' is missing the SHOW DATABASES privilege. "
+                            "This means that MaxScale cannot see all databases and authentication can fail.",
                             service->name, user);
             }