Document journald sockets paths a bit more

Adding them to the documentation makes it easier to find
the right man page for people who are trying to understand
where some socket in the filesystem is coming from.

src/journal/journald-server.c

@@ -1573,18 +1573,23 @@ int server_init(Server *s) {
         no_sockets = s->native_fd < 0 && s->stdout_fd < 0 && s->syslog_fd < 0 && s->audit_fd < 0;
 
         /* always open stdout, syslog, native, and kmsg sockets */
+
+        /* systemd-journald.socket: /run/systemd/journal/stdout */
         r = server_open_stdout_socket(s);
         if (r < 0)
                 return r;
 
+        /* systemd-journald-dev-log.socket: /run/systemd/journal/dev-log */
         r = server_open_syslog_socket(s);
         if (r < 0)
                 return r;
 
+        /* systemd-journald.socket: /run/systemd/journal/socket */
         r = server_open_native_socket(s);
         if (r < 0)
                 return r;
 
+        /* /dev/ksmg */
         r = server_open_dev_kmsg(s);
         if (r < 0)
                 return r;